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>

server/routers/settings.py

@@ -91,6 +91,7 @@ async def get_settings():
         glm_mode=_is_glm_mode(),
         ollama_mode=_is_ollama_mode(),
         testing_agent_ratio=_parse_int(all_settings.get("testing_agent_ratio"), 1),
+        playwright_headless=_parse_bool(all_settings.get("playwright_headless"), default=True),
     )
 
 
@@ -106,6 +107,9 @@ async def update_settings(update: SettingsUpdate):
     if update.testing_agent_ratio is not None:
         set_setting("testing_agent_ratio", str(update.testing_agent_ratio))
 
+    if update.playwright_headless is not None:
+        set_setting("playwright_headless", "true" if update.playwright_headless else "false")
+
     # Return updated settings
     all_settings = get_all_settings()
     return SettingsResponse(
@@ -114,4 +118,5 @@ async def update_settings(update: SettingsUpdate):
         glm_mode=_is_glm_mode(),
         ollama_mode=_is_ollama_mode(),
         testing_agent_ratio=_parse_int(all_settings.get("testing_agent_ratio"), 1),
+        playwright_headless=_parse_bool(all_settings.get("playwright_headless"), default=True),
     )