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/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>