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/lib/types.ts

@@ -529,12 +529,14 @@ export interface Settings {
   glm_mode: boolean
   ollama_mode: boolean
   testing_agent_ratio: number  // Regression testing agents (0-3)
+  playwright_headless: boolean
 }
 
 export interface SettingsUpdate {
   yolo_mode?: boolean
   model?: string
   testing_agent_ratio?: number
+  playwright_headless?: boolean
 }
 
 export interface ProjectSettingsUpdate {