Selectable caption font, bundled offline

[ankitsejwal]

What

Adds a selectable caption font (#18). Instead of relying on the inconsistent system-cursive fallback, users pick from four bundled handwriting fonts via a picker in the controls row; the choice applies live to every polaroid.

• Caveat (default), Patrick Hand, Shadows Into Light, Kalam
• Fonts self-hosted via @fontsource — no external/Google Fonts fetch, stays fully offline/private
• Selection stored in the settings store; applied to captions via font-family
• Picker <option>s render in their own font for a quick preview

Built as 8 granular commits (deps → load → registry → setting → apply → control → wire → types fix).

Verification

Check	Result
pnpm lint	✅ oxlint 0/0
pnpm typecheck	✅ tsc 7.0.1-rc
pnpm build	✅ Vite 8

Notes

• noUncheckedSideEffectImports (our strict TS setting) flagged the @fontsource CSS imports (they ship no types) — added an ambient declare module '@fontsource/*'.
• Replaced the old fixed --font-handwriting token (now dead) with the dynamic per-selection font.

Closes #18