feat: resizable sidebar, branch hover preview, Fork Point label fix

- Sidebar: click-and-drag resize handle (200-500px range), accent
  highlight on drag, cursor changes to col-resize
- BranchNode: hover/active expands to show context_summary (tried,
  result), status_reason, and step count with smooth animation
- ForkCard: "Fork Point" label changed from text-muted to
  text-accent-text for visibility against bg-card

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/session/BranchNode.tsx

@@ -1,3 +1,4 @@
+import { useState } from 'react'
 import { CircleDot, CheckCircle2, XCircle, Circle, RotateCcw } from 'lucide-react'
 import { cn } from '@/lib/utils'
 import type { BranchResponse } from '@/types/branching'
@@ -52,40 +53,86 @@ interface BranchNodeProps {
 }
 
 export function BranchNode({ branch, depth, isActive, onClick }: BranchNodeProps) {
+  const [isHovered, setIsHovered] = useState(false)
   const config = STATUS_CONFIG[branch.status]
   const Icon = config.icon
+  const showDetail = isActive || isHovered
 
   return (
-    <button
-      type="button"
-      onClick={() => onClick(branch.id)}
-      style={{ paddingLeft: `${8 + depth * 16}px` }}
-      className={cn(
-        'w-full flex items-center gap-2 py-2 pr-3 text-left rounded-md transition-colors',
-        'hover:bg-elevated',
-        isActive && 'bg-accent-dim border-l-2 border-accent'
-      )}
+    <div
+      onMouseEnter={() => setIsHovered(true)}
+      onMouseLeave={() => setIsHovered(false)}
     >
-      <Icon
-        size={14}
-        className={cn('shrink-0', config.textClass)}
-      />
-      <span
+      <button
+        type="button"
+        onClick={() => onClick(branch.id)}
+        style={{ paddingLeft: `${8 + depth * 16}px` }}
         className={cn(
-          'flex-1 text-sm truncate',
-          isActive ? 'text-heading font-medium' : 'text-primary'
+          'w-full flex items-center gap-2 py-2 pr-3 text-left rounded-md transition-all duration-150',
+          'hover:bg-elevated',
+          isActive && 'bg-accent-dim border-l-2 border-accent'
         )}
       >
-        {branch.label}
-      </span>
-      <span
+        <Icon
+          size={14}
+          className={cn('shrink-0', config.textClass)}
+        />
+        <span
+          className={cn(
+            'flex-1 text-sm truncate',
+            isActive ? 'text-heading font-medium' : 'text-primary'
+          )}
+        >
+          {branch.label}
+        </span>
+        <span
+          className={cn(
+            'text-[10px] font-semibold uppercase tracking-wider px-1.5 py-0.5 rounded-full shrink-0',
+            config.badgeClass
+          )}
+        >
+          {config.label}
+        </span>
+      </button>
+
+      {/* Expandable detail panel */}
+      <div
         className={cn(
-          'text-[10px] font-semibold uppercase tracking-wider px-1.5 py-0.5 rounded-full shrink-0',
-          config.badgeClass
+          'overflow-hidden transition-all duration-200 ease-out',
+          showDetail ? 'max-h-[120px] opacity-100' : 'max-h-0 opacity-0'
         )}
+        style={{ paddingLeft: `${24 + depth * 16}px` }}
       >
-        {config.label}
-      </span>
-    </button>
+        <div className="py-1.5 pr-3 space-y-1">
+          {branch.context_summary ? (
+            <>
+              {branch.context_summary.tried.length > 0 && (
+                <p className="text-[11px] text-muted-foreground leading-snug">
+                  <span className="text-muted">Tried:</span>{' '}
+                  {branch.context_summary.tried.join(', ')}
+                </p>
+              )}
+              {branch.context_summary.concluded && (
+                <p className="text-[11px] text-muted-foreground leading-snug">
+                  <span className="text-muted">Result:</span>{' '}
+                  {branch.context_summary.concluded}
+                </p>
+              )}
+            </>
+          ) : (
+            <p className="text-[11px] text-muted italic">No activity yet</p>
+          )}
+          {branch.status_reason && (
+            <p className="text-[11px] text-muted-foreground leading-snug">
+              <span className="text-muted">Reason:</span>{' '}
+              {branch.status_reason}
+            </p>
+          )}
+          <div className="flex items-center gap-3 text-[10px] text-muted">
+            <span>{branch.step_count} step{branch.step_count !== 1 ? 's' : ''}</span>
+          </div>
+        </div>
+      </div>
+    </div>
   )
 }

frontend/src/components/session/ForkCard.tsx

@@ -14,7 +14,7 @@ export function ForkCard({ fork, selectedBranchId, onSelectOption }: ForkCardPro
       {/* Header */}
       <div className="flex items-center gap-2">
         <GitFork size={16} className="text-accent shrink-0" />
-        <span className="text-[10px] font-semibold uppercase tracking-wider text-muted">
+        <span className="text-[10px] font-semibold uppercase tracking-wider text-accent-text">
           Fork Point
         </span>
       </div>

frontend/src/pages/DevBranchingPage.tsx

@@ -5,7 +5,7 @@
  * Renders all branching components with mock data — no API calls.
  * Use this to validate visual design before wiring into FlowPilotSessionPage.
  */
-import { useState } from 'react'
+import { useState, useCallback, useEffect, useRef } from 'react'
 import { BranchMap } from '@/components/session/BranchMap'
 import { ForkCard } from '@/components/session/ForkCard'
 import { BranchTransitionBar } from '@/components/session/BranchTransitionBar'
@@ -421,6 +421,8 @@ export default function DevBranchingPage() {
   const [showHandoff, setShowHandoff] = useState(false)
   const [previousBranchId, setPreviousBranchId] = useState<string | null>(null)
   const [showTransition, setShowTransition] = useState(false)
+  const [sidebarWidth, setSidebarWidth] = useState(280)
+  const [isResizing, setIsResizing] = useState(false)
 
   const activeBranch = MOCK_BRANCHES.find(b => b.id === activeBranchId)!
   const previousBranch = previousBranchId ? MOCK_BRANCHES.find(b => b.id === previousBranchId) ?? null : null
@@ -436,15 +438,61 @@ export default function DevBranchingPage() {
     }
   }
 
+  // ── Resizable sidebar ──
+  const sidebarRef = useRef<HTMLDivElement>(null)
+  const isResizingRef = useRef(false)
+
+  const handleMouseDown = useCallback((e: React.MouseEvent) => {
+    e.preventDefault()
+    isResizingRef.current = true
+    setIsResizing(true)
+    document.body.style.cursor = 'col-resize'
+    document.body.style.userSelect = 'none'
+  }, [])
+
+  useEffect(() => {
+    function handleMouseMove(e: MouseEvent) {
+      if (!isResizingRef.current) return
+      const newWidth = Math.min(Math.max(e.clientX, 200), 500)
+      setSidebarWidth(newWidth)
+    }
+    function handleMouseUp() {
+      if (!isResizingRef.current) return
+      isResizingRef.current = false
+      setIsResizing(false)
+      document.body.style.cursor = ''
+      document.body.style.userSelect = ''
+    }
+    document.addEventListener('mousemove', handleMouseMove)
+    document.addEventListener('mouseup', handleMouseUp)
+    return () => {
+      document.removeEventListener('mousemove', handleMouseMove)
+      document.removeEventListener('mouseup', handleMouseUp)
+    }
+  }, [])
+
   return (
     <div className="flex h-full">
       {/* Branch Map Sidebar */}
-      <aside className="w-[260px] shrink-0 border-r border-default bg-sidebar p-3 overflow-y-auto">
+      <aside
+        ref={sidebarRef}
+        style={{ width: sidebarWidth }}
+        className="shrink-0 border-r border-default bg-sidebar p-3 overflow-y-auto relative"
+      >
         <BranchMap
           branches={MOCK_BRANCHES}
           activeBranchId={activeBranchId}
           onSelectBranch={handleSwitchBranch}
         />
+        {/* Resize handle */}
+        <div
+          onMouseDown={handleMouseDown}
+          className={cn(
+            'absolute top-0 right-0 w-1 h-full cursor-col-resize transition-colors',
+            'hover:bg-accent/40',
+            isResizing ? 'bg-accent/60' : 'bg-transparent'
+          )}
+        />
       </aside>
 
       {/* Main Content */}