Terminal renders CJK (Japanese/Chinese/Korean) text as black boxes — WebGL renderer + Latin-only fontFamily

[mhit]

Describe the bug

In the built-in terminal (Shell view), CJK characters (Japanese / Chinese / Korean)
are frequently rendered as solid black boxes (□ "tofu") instead of the real glyphs.

The same Japanese text renders correctly everywhere else in the app (chat panel,
sidebar, etc.), so this is specific to the xterm.js terminal — not the OS fonts.

The breakage is per-glyph / intermittent: most CJK in a line renders, but common
kana/kanji (e.g. ク, レ, 差, 正) randomly turn into boxes. That pattern is the
signature of a font-fallback / glyph-atlas problem in the WebGL renderer, not of
missing OS fonts (a real coverage gap would drop all CJK, not a random subset).

Steps to reproduce

1. Open the terminal and run a command that prints CJK, e.g. echo 日本語テスト 中文 한국어
   (or run any CLI that emits Japanese/Chinese/Korean output).
2. Some CJK characters render as solid black boxes.

Screenshots

Environment

• claudecodeui: main, self-hosted
• Browser: Brave (Chromium) on Linux / Wayland
• OS CJK fonts installed (noto-fonts-cjk); identical text renders fine in the
  non-terminal UI, confirming the OS has the glyphs.

Root cause (from source)

Two compounding issues under src/components/shell/:

1. fontFamily has no CJK font — src/components/shell/constants/constants.ts:

   export const TERMINAL_OPTIONS: ITerminalOptions = {
     // ...
     fontFamily: 'Menlo, Monaco, "Courier New", monospace',
     // ...
   };

   Menlo / Monaco / Courier New have no CJK coverage, so CJK can only come from the
   generic monospace fallback.

2. WebGL renderer is loaded unconditionally — src/components/shell/hooks/useShellTerminal.ts:

   try {
     nextTerminal.loadAddon(new WebglAddon());
   } catch {
     console.warn('[Shell] WebGL renderer unavailable, using Canvas fallback');
   }
   The xterm.js WebGL (and canvas) renderer rasterizes glyphs into a texture atlas and
   handles *fallback* glyphs (those not in the specified fonts) poorly — CJK obtained
   via the generic `monospace` fallback frequently comes out as the missing-glyph box.
   The DOM renderer doesn't have this issue because it uses native browser text
   rendering / font fallback.

Suggested fix

• Minimal: add cross-platform CJK monospace fonts to TERMINAL_OPTIONS.fontFamily,
  giving the atlas an explicit CJK font to rasterize from:

  fontFamily:
    'Menlo, Monaco, "Courier New", "Noto Sans Mono CJK JP", "Noto Sans CJK JP", ' +
    'Meiryo, "MS Gothic", "Microsoft YaHei", "PingFang SC", "Hiragino Sans", monospace',

• Robust: make fontFamily user-configurable (relates to Feature/font customization #730) and/or expose an
  option to disable the WebGL addon (fall back to the DOM renderer), which renders CJK
  reliably even with an imperfect font stack.