Fix synced local tweaks

[eleboucher]

Description

Restores locally imported CSS tweaks after settings sync on a new device. The change migrates legacy cache-only tweaks, restores listing and application without IndexedDB races, bounds aggregate CSS payloads, preserves source-only oversized tweaks, and reports partial sync when necessary.

Verification

• npm run test:run -- src/app/utils/settingsSync.test.ts src/app/hooks/useSettingsSync.test.tsx src/app/theme/syncedTweakCss.test.ts src/app/pages/ThemeManager.test.tsx src/app/theme/processThemeImport.test.ts src/app/features/settings/cosmetics/ThemeCatalogSettings.test.tsx (65 passed)
• npm run fmt:check -- src/app/utils/settingsSync.ts src/app/utils/settingsSync.test.ts
• git diff --check
• npm run typecheck remains blocked by pre-existing Matrix OAuth exports and the missing sharekit declaration; no errors point to this change.

Fixes #1208

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).