[Feature Request]: Add font size & font family options (English + CJK) in Desktop GUI

Is your feature request related to a problem?

Yes. The Desktop GUI has no font customization at all — font size is fixed and there is no way to choose a font. For a coding tool whose UI is dominated by text (conversation, code, diffs, terminals), this hurts readability:

• Font size: On high-DPI Windows displays the default size is too small to read comfortably; on some setups it is too large.
• Font family: There is no option to pick a preferred font. English UI benefits from monospace/humanist fonts (e.g. Cascadia Code, JetBrains Mono, Inter), while Chinese users often want a specific CJK font (e.g. Microsoft YaHei, Noto Sans SC, Sarasa Mono SC) for mixed Chinese/English text — the default fallback is often inconsistent.

Describe the solution you'd like

Add a Fonts & Size section under Settings → Appearance:

1. Font size: A slider or numeric input (e.g. 12–24 px) controlling UI font size, with live preview and persistence.
2. Font family (English): A dropdown of common fonts — e.g. Inter, Segoe UI, JetBrains Mono, Cascadia Code, Fira Code — plus an "custom font family" text field for power users.
3. Font family (Chinese/CJK): A separate dropdown for CJK fonts — e.g. Microsoft YaHei, PingFang SC (macOS), Noto Sans SC, Sarasa Mono SC — applied via the CSS font-family fallback chain so mixed Chinese/English text renders consistently.
4. Optional: separate monospace font setting for code blocks / terminal panes (already partially used via Monaco/xterm), so users can match their editor fonts.

The implementation is a lightweight CSS-variables + font-family/font-size mapping in desktop/src/**; no backend change is required.

Describe alternatives you've considered

• A single "font size" zoom toggle (Ctrl +/-) — quick win but no font selection.
• Letting users edit a CSS override file — flexible but not beginner-friendly.

Additional context

• Platform: Windows 11, 2560×1440 display (high-DPI).
• VS Code users are used to editor.fontFamily/editor.fontSize; matching this mental model makes adoption easier.
• Version: DeepCode Desktop 0.1.0 (v2.0.0 runtime).