feat(ui): replace Switch with TogglePill per design audit (#687)

* chore: ignore local design-system references

Adds frontend/DESIGN.md and .design-bundle/ to .gitignore so the
design-system reference and the audit handoff bundle stay local
to each contributor's machine.

* feat(ui): replace Switch with TogglePill per design audit

Adds a mono-uppercase ON/OFF pill that replaces the sliding Switch
across every call site. The pill uses role="switch" plus aria-checked,
tints success-green when on and neutral card-tone when off, and holds
a stable 60px min-width so the label flip does not shift layout. It
reinforces the tracked-mono uppercase pattern already used for kicker
labels and table column headers, and echoes the UP/DN status language
of the sidebar stack list.

- Add TogglePill component (frontend/src/components/ui/toggle-pill.tsx)
- Replace 17 Switch call sites across 10 files (settings sections,
  SSO providers, scheduled operations, network topology, resources,
  stack auto-heal)
- Remove unused Switch wrapper and radix primitive
- Drop the @radix-ui/react-switch dependency

frontend/src/components/settings/DeveloperSection.tsx

@@ -1,7 +1,7 @@
 import { Button } from '@/components/ui/button';
 import { Input } from '@/components/ui/input';
 import { Label } from '@/components/ui/label';
-import { Switch } from '@/components/ui/switch';
+import { TogglePill } from '@/components/ui/toggle-pill';
 import { Skeleton } from '@/components/ui/skeleton';
 import { Badge } from '@/components/ui/badge';
 import { Tooltip, TooltipContent, TooltipProvider, TooltipTrigger } from '@/components/ui/tooltip';
@@ -64,10 +64,10 @@ export function DeveloperSection({ settings, onSettingChange, onSave, isSaving,
                                 <Label htmlFor="developer_mode" className="text-base">Developer Mode</Label>
                                 <p className="text-xs text-muted-foreground">Enable Real-Time Metrics, Debug Diagnostics & Extended Logs</p>
                             </div>
-                            <Switch
+                            <TogglePill
                                 id="developer_mode"
                                 checked={settings.developer_mode === '1'}
-                                onCheckedChange={(c) => onSettingChange('developer_mode', c ? '1' : '0')}
+                                onChange={(c) => onSettingChange('developer_mode', c ? '1' : '0')}
                             />
                         </div>