feat(theme): implement theme persistence, JSON parser, and IndexedDB v6 (#126)

[Dyu20705]

Summary

Implements Issue #126: Theme Import, Parser, and IndexedDB Persistence.

Changes

1. IndexedDB v6 Migration: Upgrades DB_VERSION from 5 to 6 in core/storage.js with additive creation of userThemes (keyPath: 'id') and settings (keyPath: 'key') stores.
2. Settings & User Theme Storage API: Implements getSettings, putSettings, saveUserThemeToDb, getUserThemeFromDb, listUserThemesFromDb, and deleteUserThemeFromDb in core/storage.js.
3. Theme Storage Abstraction: Implements core/theme/themeStorage.js providing safe JSON theme import with 256KB length bounds and schema validation, JSON export, unified theme listing (merging built-ins with user themes), CRUD operations, and immutability protection for built-in themes.
4. Database Snapshot Roundtrip: Updates exportDatabase and importDatabase in core/storage.js to guarantee 100% lossless backup/restore of custom themes and user settings.
5. Test Coverage: Adds tests/integration/storage.theme.test.mjs and updates existing storage and E2E version assertions to v6.

Verification

• npm run test:content: PASS (3 tests)
• npm run lint: PASS (0 errors)
• npm run test:unit: PASS (254 tests)
• npm run test:integration: PASS (107 tests across 11 suites)
• npm run test:perf: PASS (4 budget checks)
• npm run test:e2e: PASS (118 Playwright tests)
• git diff --check: Clean

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