feat(settings): add font size customization for chat message bubbles

[Antra1705]

🔗 Related Issue

Closes #425

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📝 What does this PR do?

Adds a user-configurable font size setting for chat message bubbles. Users can select Small, Medium, or Large from a new "Chat Typography" section in the Settings page. The preference is persisted via localStorage and applied reactively without a page reload.

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🗂️ Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🔧 Refactor / code cleanup
• 📝 Documentation update
• 🎨 UI / styling change
• ⚙️ CI / tooling / config change
• 🧪 Tests

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🧪 How was this tested?

• Ran the frontend locally (npm run dev inside frontend/)
• Added / updated tests

Ran npx vitest run — all 5 tests passed (SourceCard: 3, MessageBubble: 2). Ran npm run lint — 0 errors, 15 warnings (all pre-existing, none introduced by this PR). Ran next build — compiled successfully with no TypeScript errors.

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📸 Screenshots (if UI change)

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⚠️ Anything to flag for reviewers?

• Used useSyncExternalStore instead of useState + useEffect for hydration safety — avoids cascading renders and complies with react-hooks/set-state-in-effect ESLint rule.
• Font size mapping preserves existing baseline: small → text-xs, medium → text-sm (default, matches current), large → text-base.
• The 15 lint warnings are pre-existing in the codebase and not introduced by this PR.

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✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed

[github-actions]

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