feat(electrobun): settings overhaul — fonts, shells, providers, retention, chords

- Settings drawer: responsive width clamp(24rem, 45vw, 50rem)
- System font detection: fc-list for UI fonts (preferred sans-serif starred)
  and mono fonts (Nerd Fonts starred), fallback to hardcoded lists
- Scrollback: default 5000, min 1000, step 500
- Shell detection: system.shells RPC, pre-selects $SHELL login shell
- Provider enablement: provider.scan gates toggle, unavailable shown as N/A
- Session retention: count 0-100 (0=Keep all), age 0-365 (0=Forever)
- Chord keybindings: Ctrl+K → Ctrl+S style multi-key sequences,
  1s prefix wait, arrow separator display, 26 tests passing

ui-electrobun/src/shared/pty-rpc-schema.ts

@@ -255,7 +255,16 @@ export type PtyRPCRequests = {
   /** Detect available shells on this system. */
   "system.shells": {
     params: Record<string, never>;
-    response: { shells: Array<{ path: string; name: string }> };
+    response: { shells: Array<{ path: string; name: string }>; loginShell: string };
+  };
+
+  /** Detect installed system fonts (uses fc-list). */
+  "system.fonts": {
+    params: Record<string, never>;
+    response: {
+      uiFonts: Array<{ family: string; preferred: boolean }>;
+      monoFonts: Array<{ family: string; isNerdFont: boolean }>;
+    };
   };
 
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