feat: refactor scratchpad to floating overlay with global thin scrollbars

Refactor scratchpad from a flex-layout sidebar that pushes content left
to a floating overlay panel (position: fixed) that doesn't affect layout.
Panel slides in from the right with Ctrl+/ toggle. Main content adjusts
padding responsively when panel is open.

Also apply thin scrollbar styling globally across all scrollable elements
for a consistent, minimal look.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/plans/2026-02-04-scratchpad-overlay-design.md

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+# Scratchpad Floating Overlay Design
+
+> **Date:** 2026-02-04
+> **Status:** Approved
+
+---
+
+## Problem
+
+The scratchpad is a full-height sidebar that pushes main content left when open, reducing the available width for tree navigation. On smaller screens this is particularly disruptive. It should be a floating overlay that doesn't affect layout.
+
+## Design
+
+### Component: ScratchpadSidebar.tsx
+
+Single component with two visual states, both using `position: fixed`.
+
+**Closed — floating button:**
+- Fixed to viewport right edge, vertically centered
+- `right: 0; top: 50%; transform: translateY(-50%); z-index: 40`
+- Rounded left corners, flat right edge (`rounded-l-md`)
+- StickyNote icon
+- Amber dot (absolute positioned) when unsaved changes exist
+
+**Open — overlay panel:**
+- Fixed to viewport right edge, vertically centered
+- `right: 0; top: 50%; transform: translateY(-50%); z-index: 40`
+- Width: 420px, height: 55vh
+- Rounded left corners, flat right edge (`rounded-l-lg`)
+- Shadow (`shadow-xl`) and left border for depth
+- Slide-in animation: `translateX(100%)` → `translateX(0)`, 200ms ease-out
+- No backdrop — clicking outside does NOT close it
+
+**Panel contents (unchanged):**
+- Header: title, preview toggle (Eye/Pencil), close button (X)
+- Body: textarea or markdown preview, scrollable
+- Footer: save status indicator
+
+### Keyboard Shortcut
+
+`Ctrl+/` toggles open/closed. Handled via `useEffect` keydown listener inside the component.
+
+### TreeNavigationPage.tsx
+
+- Outer wrapper no longer uses `flex` for sidebar layout
+- Main content takes full width
+- ScratchpadSidebar renders in same DOM position (fixed positioning makes it layout-independent)
+
+### Preserved Behavior
+
+- Auto-save with 1000ms debounce
+- Markdown preview toggle
+- Save status indicators (Saving.../Saved/Unsaved changes)
+- localStorage persistence for open/closed state (key: `scratchpad-collapsed`)
+- beforeunload warning for unsaved changes
+- Session ID reset on navigation
+
+## Files Changed
+
+| File | Change |
+|------|--------|
+| `frontend/src/components/session/ScratchpadSidebar.tsx` | Refactor to fixed-position floating overlay |
+| `frontend/src/pages/TreeNavigationPage.tsx` | Remove flex sidebar layout |