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/styles.css

@@ -16,6 +16,19 @@
   font-weight: 400 700;
   src: url('fonts/jetbrains-mono-variable.woff2') format('woff2');
 }
+/* Icons-only per-glyph fallback for the terminal (Symbols Nerd Font Mono, MIT,
+   fonts/LICENSE-nerd-fonts.txt). Sits BEHIND the text fonts in the xterm stack
+   (constants.js: TERMINAL_FONT_DEFAULT_STACK), so it only ever supplies the
+   private-use-area glyphs shell prompts draw (powerline, p10k/starship folder
+   and git icons) — text rendering is untouched. `block` display, not `swap`:
+   there is no fallback that CAN render these glyphs, so swapping in tofu first
+   would poison xterm's glyph atlas until the next re-render. */
+@font-face {
+  font-family: 'Symbols Nerd Font Mono';
+  font-style: normal;
+  font-display: block;
+  src: url('fonts/symbols-nerd-font-mono.woff2') format('woff2');
+}
 
 :root {
   /* Dark is the safe fallback. Light skins override this so native selects,