Implement stat block panel fold/unfold and pin-to-second-panel

Replace the close button and heading with fold/unfold controls that
collapse the panel to a slim right-edge tab showing the creature name
vertically, and add a pin button (xl+ viewports with creature loaded)
that opens the creature in a second left-side panel for simultaneous
reference. Fold state is respected on turn change. 19 acceptance tests.

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

specs/035-statblock-fold-pin/quickstart.md

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+# Quickstart: Stat Block Panel Fold/Unfold and Pin
+
+**Feature**: 035-statblock-fold-pin
+**Date**: 2026-03-11
+
+## What This Feature Does
+
+Replaces the stat block panel's close button with fold/unfold controls and adds the ability to pin a creature to a second panel for side-by-side reference.
+
+## Key Files to Modify
+
+| File | Change |
+|------|--------|
+| `apps/web/src/components/stat-block-panel.tsx` | Core changes: remove close button + heading, add fold/unfold toggle, add pin/unpin buttons, add folded tab view, accept new props for panel role |
+| `apps/web/src/App.tsx` | New state: `pinnedCreatureId`, `isRightPanelFolded`. Render second StatBlockPanel for pinned creature. Update auto-show logic to unfold panel. |
+| `apps/web/src/index.css` | New CSS: folded tab styles, slide transition utility, vertical text utility |
+
+## New Files
+
+None expected. All changes fit within existing component structure.
+
+## Architecture Notes
+
+- **Layer**: All changes are in the Adapter (web) layer — no domain or application changes needed.
+- **State**: Ephemeral React state only — no persistence changes.
+- **Component reuse**: The same `StatBlockPanel` component serves both browse (right) and pinned (left) panels, differentiated by a prop.
+- **Animation**: CSS transitions on `translate` property, matching existing 200ms ease-out pattern.
+
+## Dev Commands
+
+```bash
+pnpm --filter web dev          # Start dev server to test visually
+pnpm vitest run apps/web/src   # Run web app tests
+pnpm check                     # Full quality gate before commit
+```
+
+## Testing Strategy
+
+- Unit tests for fold/unfold state transitions and conditional rendering
+- Unit tests for pin/unpin behavior and viewport-responsive pin button visibility
+- Visual testing via dev server at various viewport widths (< 1024px, 1024-1279px, 1280px+)