feat: implement font size preferences

[chenyme]

Summary

Add a global font size preference for the app. Users can choose Small, Standard, Medium, or Large, and the setting scales both general UI text and chat content through global CSS variables instead of changing individual components.

The preference is persisted in appearance preferences, synchronized across account/local settings, and safely normalized by the backend so invalid or legacy font size values fall back to the standard size instead of causing profile save failures.

Change type

• Bug fix
• Feature
• Documentation
• Refactor
• Configuration / deployment
• Security hardening
• Other

Affected areas

• Frontend / UI
• Backend / API
• Authentication / authorization
• Conversations / streaming
• Files / RAG / extraction
• Model routing / providers
• MCP / tools
• Billing / payments
• Admin console
• Deployment / Docker / configuration
• Documentation

Verification

• cd frontend && pnpm lint
• cd backend && go test -count=1 ./internal/application/auth
• git diff --check

Screenshots, API examples, or logs

N/A

Configuration, migration, and compatibility notes

No database migration is required. The setting is stored in the existing appearancePreferences JSON field.

Invalid or unknown fontSize values are normalized to standard to avoid profile save failures after upgrades or stale client state.

Documentation

• Documentation is not needed for this change.
• Documentation was updated.
• Documentation still needs to be updated.

Security and privacy

• No secrets, tokens, credentials, local config, or personal data are included.
• User data access remains scoped by authenticated user context unless an admin-only path explicitly requires broader access.
• Security-sensitive behavior was reviewed, including authentication, authorization, provider routing, file processing, billing, and admin APIs where relevant.

Checklist

• I searched existing issues and pull requests.
• Changes are focused and do not include unrelated refactors.
• Tests or static verification were run where practical.
• User-facing behavior, deployment steps, API contracts, or configuration changes are documented.
• Generated artifacts are included only when this project explicitly requires them.
• Caches, build output, .pyc files, .env files, and local storage data are not committed.