feat(views): add grid view with 2D keyboard navigation and view mode persistence (#131)

[Dyu20705]

Issue relationships

Parent issue: #128
Depends on: #130 (PR #153 merged)
Blocks: #132

Problem and scope

Implements Issue #131: Grid View and Custom Reordering.

• Implements visual Grid View alongside the standard List View for note browsing.
• Adds #viewModeButton in note panel header actions to toggle between List and Grid modes.
• Persists user view mode preference in IndexedDB settings store (settings.app.viewMode).
• Implements 2D keyboard navigation across grid cards (ArrowUp, ArrowDown, ArrowLeft, ArrowRight, Home, End, Enter).
• Registers notes.toggle-view-mode command bound to Ctrl+Shift+G / Cmd+Shift+G.
• Preserves virtualized and bounded DOM performance invariants.

Verification evidence

npm run test:content     -> PASS (3/3 passed)
npm run lint             -> PASS (0 errors, 0 warnings)
npm run test:unit        -> PASS (289/289 passed)
npm run test:integration -> PASS (107/107 passed)
npm run test:perf        -> PASS (4/4 passed)
git diff --check         -> PASS (clean)

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