feat: bundled Nerd Font symbols fallback + per-device terminal font setting

Shell prompts using Nerd Font glyphs (powerline, p10k/starship folder and
git icons) rendered as missing-glyph boxes: the built-in xterm stack has no
private-use-area symbols, and phones have no Nerd Fonts installed at all.

- Bundle Symbols Nerd Font Mono (icons-only, MIT, 1.2MB woff2) served from
  fonts/ and appended to the terminal stack before monospace — browsers fall
  back per glyph, so icons render everywhere while text stays in the text
  fonts. font-display: block + preload keep tofu out of xterm's glyph atlas.
- New per-device terminalFontFamily setting (App Settings > Terminal &
  Input > Font): prepended to the built-in stack, never a replacement, so
  the symbols fallback and final monospace always survive. Applied live on
  save (refit + echo-overlay refreshFont, mirroring setFontSize).
- Single source for both xterm surfaces: TERMINAL_FONT_DEFAULT_STACK +
  resolveTerminalFontFamily() in constants.js, unit-tested.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VjnbbZRBuvR5E3SDouwXr9

src/web/public/i18n.js

@@ -326,6 +326,10 @@
 
     // Input settings
     Input: '输入',
+    Font: '字体',
+    'Terminal font': '终端字体',
+    'Prepended to the built-in stack, so fallbacks (including bundled Nerd Font symbols) keep working. Must be installed on this device. Leave empty for the default.':
+      '置于内置字体栈之前，回退字体（包括内置的 Nerd Font 图标）仍然生效。需已安装在本设备上。留空使用默认值。',
     'Local Echo': '本地回显',
     'CJK Input': '中日韩输入',
     'Extended Keyboard Bar': '扩展键盘栏',