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/mainview/settings/AppearanceSettings.svelte

@@ -8,7 +8,8 @@
   import ThemeEditor from './ThemeEditor.svelte';
   import CustomDropdown from '../ui/CustomDropdown.svelte';
 
-  const UI_FONTS = [
+  // Fallback lists — replaced by system detection on mount
+  const FALLBACK_UI_FONTS = [
     { value: '',               label: 'System Default' },
     { value: 'Inter',          label: 'Inter' },
     { value: 'IBM Plex Sans',  label: 'IBM Plex Sans' },
@@ -17,7 +18,7 @@
     { value: 'Ubuntu',         label: 'Ubuntu' },
   ];
 
-  const TERM_FONTS = [
+  const FALLBACK_TERM_FONTS = [
     { value: '',                label: 'Default (JetBrains Mono)' },
     { value: 'JetBrains Mono', label: 'JetBrains Mono' },
     { value: 'Fira Code',      label: 'Fira Code' },
@@ -27,6 +28,9 @@
     { value: 'monospace',      label: 'monospace' },
   ];
 
+  let uiFontItems  = $state(FALLBACK_UI_FONTS.map(f => ({ value: f.value, label: f.label })));
+  let termFontItems = $state(FALLBACK_TERM_FONTS.map(f => ({ value: f.value, label: f.label })));
+
   // ── Local reactive state ───────────────────────────────────────────────────
   let themeId      = $state<ThemeId>(themeStore.currentTheme);
   let uiFont       = $state(fontStore.uiFontFamily);
@@ -35,7 +39,7 @@
   let termFontSize = $state(fontStore.termFontSize);
   let cursorStyle  = $state('block');
   let cursorBlink  = $state(true);
-  let scrollback   = $state(1000);
+  let scrollback   = $state(5000);
 
   interface CustomThemeMeta { id: string; name: string; }
   let customThemes = $state<CustomThemeMeta[]>([]);
@@ -67,8 +71,7 @@
 
   // ── Dropdown items for CustomDropdown ──────────────────────────────────────
   let themeItems = $derived(allThemes.map(t => ({ value: t.id, label: t.label, group: t.group })));
-  let uiFontItems = UI_FONTS.map(f => ({ value: f.value, label: f.label }));
-  let termFontItems = TERM_FONTS.map(f => ({ value: f.value, label: f.label }));
+  // uiFontItems and termFontItems are $state — populated by system.fonts on mount
   let langItems = AVAILABLE_LOCALES.map(l => ({ value: l.tag, label: l.nativeLabel }));
 
   // ── Actions ────────────────────────────────────────────────────────────────
@@ -135,10 +138,35 @@
     const { settings } = await appRpc.request['settings.getAll']({}).catch(() => ({ settings: {} }));
     if (settings['cursor_style']) cursorStyle = settings['cursor_style'];
     if (settings['cursor_blink']) cursorBlink = settings['cursor_blink'] !== 'false';
-    if (settings['scrollback'])  scrollback  = parseInt(settings['scrollback'], 10) || 1000;
+    if (settings['scrollback'])  scrollback  = parseInt(settings['scrollback'], 10) || 5000;
 
     const res = await appRpc.request['themes.getCustom']({}).catch(() => ({ themes: [] }));
     customThemes = res.themes.map(t => ({ id: t.id, name: t.name }));
+
+    // Detect system fonts
+    try {
+      const fontRes = await appRpc.request['system.fonts']({});
+      if (fontRes.uiFonts.length > 0) {
+        uiFontItems = [
+          { value: '', label: 'System Default' },
+          ...fontRes.uiFonts.map(f => ({
+            value: f.family,
+            label: f.preferred ? `\u2605 ${f.family}` : f.family,
+          })),
+        ];
+      }
+      if (fontRes.monoFonts.length > 0) {
+        termFontItems = [
+          { value: '', label: 'Default (JetBrains Mono)' },
+          ...fontRes.monoFonts.map(f => ({
+            value: f.family,
+            label: f.isNerdFont ? `\u2B50 ${f.family}` : f.family,
+          })),
+        ];
+      }
+    } catch {
+      // Keep fallback font lists
+    }
   });
 </script>
 
@@ -216,8 +244,8 @@
 
   <h3 class="sh">{t('settings.scrollback')}</h3>
   <div class="field row">
-    <input type="number" class="num-in" min="100" max="100000" step="100" value={scrollback}
-      onchange={e => persistScrollback(parseInt((e.target as HTMLInputElement).value, 10) || 1000)}
+    <input type="number" class="num-in" min="1000" max="100000" step="500" value={scrollback}
+      onchange={e => persistScrollback(parseInt((e.target as HTMLInputElement).value, 10) || 5000)}
       aria-label="Scrollback lines" />
     <span class="hint">{t('settings.scrollbackHint')}</span>
   </div>