feat(settings): support custom font families and weights

[TwinklerG]

Closes #1073

Summary

Extend the existing font-size controls with custom font families and base weights in Settings → General → Appearance.

• Configure interface/chat and monospace (code, tool output, terminal) fonts independently, with comma-separated font-name lists, weights from 100–900, live previews, and separate resets.
• Persist preferences in the current browser, restore them on app startup, and synchronize them across tabs without changing chat width or font size.
• Use fonts installed on the browser's device and retain the default fallback stacks. Custom font settings do not upload or download fonts; font-name input is treated as data rather than arbitrary CSS.
• Preserve explicit heading and emphasis weights, apply monospace weights to code/source/diff views, and remeasure existing chat drafts when the interface font changes.
• Update and refit existing terminals without restarting their shells or SSE streams.
• Add English, Simplified Chinese, and Traditional Chinese labels, documentation, unit tests, and browser regression coverage.

Validation

• node_modules/.bin/tsc --noEmit --incremental false
• npm run lint
• 121 targeted tests passed, covering the font preference helpers/store and affected chat, settings, code-view, and terminal components.
• git diff --cached --check before committing.
• npm run test:e2e — passed locally in Chromium at desktop (1280px) and mobile (390px) widths. Includes font families and weights, persistence, cross-tab synchronization, independent resets, preserved emphasis, and draft resizing.
• npm run test:terminal — passed locally in Chromium at desktop (1440px) and mobile (390px) widths. Includes live terminal font/weight updates without shell or SSE restarts, persistence after refresh, and terminal lifecycle regressions.

The initial local E2E attempt was blocked by a stale .next/dev/lock. After confirming that its recorded process had exited and no Next dev server was running, the stale file was backed up and both suites completed successfully. No feature-code changes were needed.