feat: add headless browser toggle to settings UI

Replace the PLAYWRIGHT_HEADLESS environment variable with a global
setting toggle in the Settings modal. The setting is persisted in the
registry DB and injected as an env var into agent subprocesses, so
client.py reads it unchanged.

Backend:
- Add playwright_headless field to SettingsResponse/SettingsUpdate schemas
- Read/write the setting in settings router via existing _parse_bool helper
- Pass playwright_headless from agent router through to process manager
- Inject PLAYWRIGHT_HEADLESS env var into subprocess environment

Frontend:
- Add playwright_headless to Settings/SettingsUpdate TypeScript types
- Add "Headless Browser" Switch toggle below YOLO mode in SettingsModal
- Add default value to DEFAULT_SETTINGS in useProjects

Also fix CSS build warning: change @import url("tw-animate-css") to bare
@import "tw-animate-css" so Tailwind v4 inlines it during compilation
instead of leaving it for Vite/Lightning CSS post-processing.

Remove stale summary.md from previous refactoring session.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/components/SettingsModal.tsx

@@ -171,6 +171,24 @@ export function SettingsModal({ isOpen, onClose }: SettingsModalProps) {
               />
             </div>
 
+            {/* Headless Browser Toggle */}
+            <div className="flex items-center justify-between">
+              <div className="space-y-0.5">
+                <Label htmlFor="playwright-headless" className="font-medium">
+                  Headless Browser
+                </Label>
+                <p className="text-sm text-muted-foreground">
+                  Run browser without visible window (saves CPU)
+                </p>
+              </div>
+              <Switch
+                id="playwright-headless"
+                checked={settings.playwright_headless}
+                onCheckedChange={() => updateSettings.mutate({ playwright_headless: !settings.playwright_headless })}
+                disabled={isSaving}
+              />
+            </div>
+
             {/* Model Selection */}
             <div className="space-y-2">
               <Label className="font-medium">Model</Label>