feat: session quick wins (#51-#55) (#72)

* feat: add session quick wins (#51-#55)

- Session timer showing elapsed time in header (#51)
- Tab keyboard shortcut to focus notes textarea (#52)
- Repeat Last Session button on tree library page (#53)
- Auto-recovery banner for incomplete sessions (#54)
- Copy individual step to clipboard on session detail (#55)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: add missing delete button to table and list tree views

The onDeleteTree prop was accepted but never used in TreeTableView and
TreeListView. Now both views show a trash icon (permission-gated) matching
the existing grid view behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/hooks/useKeyboardShortcuts.ts

@@ -82,6 +82,13 @@ export function useTreeNavigationShortcuts({
       handler: onContinue,
       enabled: canContinue,
     },
+    // Tab to focus notes
+    {
+      key: 'Tab',
+      handler: () => {
+        document.getElementById('session-notes')?.focus()
+      },
+    },
   ]
 
   useKeyboardShortcuts(shortcuts)

frontend/src/hooks/useSessionTimer.ts

@@ -0,0 +1,33 @@
+import { useState, useEffect, useRef } from 'react'
+
+export function useSessionTimer(startedAt: string | undefined | null): string | null {
+  const [elapsed, setElapsed] = useState<string | null>(null)
+  const intervalRef = useRef<ReturnType<typeof setInterval> | null>(null)
+
+  useEffect(() => {
+    if (!startedAt) {
+      setElapsed(null)
+      return
+    }
+
+    const startTime = new Date(startedAt).getTime()
+
+    const tick = () => {
+      const diff = Math.max(0, Math.floor((Date.now() - startTime) / 1000))
+      const hours = Math.floor(diff / 3600)
+      const minutes = Math.floor((diff % 3600) / 60)
+      const seconds = diff % 60
+      const pad = (n: number) => String(n).padStart(2, '0')
+      setElapsed(hours > 0 ? `${pad(hours)}:${pad(minutes)}:${pad(seconds)}` : `${pad(minutes)}:${pad(seconds)}`)
+    }
+
+    tick()
+    intervalRef.current = setInterval(tick, 1000)
+
+    return () => {
+      if (intervalRef.current) clearInterval(intervalRef.current)
+    }
+  }, [startedAt])
+
+  return elapsed
+}