feat(perf): lazy module loading and large board grid virtualization (#145)

[Dyu20705]

Problem & Root Cause

• Previously, note board virtualization in ui/list.js and styles.css collapsed Grid View into a single column whenever virtualization was active (data-virtualized="true"), preventing multi-column responsive grid layout for collections with >500 items.
• Secondary modules like Kanji stroke guidance and dictionary assets were embedded eagerly in the main bundle.

Implementation

1. Multi-Column Grid Virtualization (ui/list.js):
   • Implemented dynamic column calculation (computeGridColumns) derived from container width and minimum card width (280px).
   • Updated renderWindow() with row-based slicing: startIndex = startRow * cols, endIndex = endRow * cols, and spacer heights computed via ceil(N / cols).
   • Preserved single-column List View virtualization.
2. Stylesheet Hardening (styles.css):
   • Removed the single-column override .note-list[data-virtualized="true"] .note-board-grid, enabling .note-list[data-view-mode="grid"].
3. Lazy Module Loading (core/kanjiStrokeGuide.js & ui/kanjiInkView.js):
   • Extracted stroke guidance rendering, direction arrows, air path calculations, and Kanji dictionary metadata into core/kanjiStrokeGuide.js.
   • Updated ui/kanjiInkView.js to dynamically load kanjiStrokeGuide.js on demand only when guidance is toggled.
4. Test Suite Expansion:
   • Added unit test suite tests/unit/kanjiStrokeGuide.test.mjs.
   • Added large board stress and dynamic import performance tests in tests/performance/performance-budget.test.mjs.
   • Added multi-column grid virtualization E2E test with 600 items in tests/e2e/grid-view.spec.mjs.

Verification Evidence

• npm run test:content -> PASS (3/3)
• npm run lint -> PASS (0 errors, 0 warnings)
• npm run test:unit -> PASS (320/320)
• npm run test:integration -> PASS (111/111)
• npm run test:perf -> PASS (6/6)
• npm run test:e2e -> PASS (165/165)
• git diff --check -> PASS

Fixes #145
Part of #143

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