feat: persistent metadata panel, symmetric sidebar toggles, keyboard nav scroll

- Right sidebar stays open by default and shows an empty state when
  nothing is selected, instead of auto-hiding on deselect.
- Both sidebars now have a collapse button in their header and an
  expand button in the TopBar that only appears when collapsed, so
  each panel has a discoverable affordance in either state.
- Arrow-key navigation auto-scrolls the destination row into view
  with a ~35% peek margin, cueing the user that there's more content
  in the scroll direction.
- Fix: the width sentinel's measurement effect never installed its
  ResizeObserver when Timeline first rendered the loading state (ref
  was null, empty-dep effect didn't re-run), so containerWidth stuck
  at 0 and the grid fell back to 4 columns × 200px forever. Switched
  to a callback ref that attaches the observer the moment the
  sentinel actually mounts.
- KeyboardHints surface the Tab (library) and I (info) shortcuts.

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

frontend/src/components/KeyboardHints.tsx

@@ -14,13 +14,14 @@ export function KeyboardHints() {
         { key: 'P', action: 'Pick → heap' },
         { key: 'X', action: 'Discard' },
         { key: 'Space', action: 'Preview' },
+        { key: 'I', action: 'Info panel' },
         { key: 'Esc', action: 'Deselect' },
       ]
     : [
         { key: '↑↓←→', action: 'Navigate' },
-        { key: 'Click', action: 'Select' },
-        { key: 'Shift+Click', action: 'Range' },
         { key: 'Space', action: 'Preview' },
+        { key: 'Tab', action: 'Library panel' },
+        { key: 'I', action: 'Info panel' },
         { key: '/', action: 'Search' },
       ]