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/schemas.py

@@ -398,6 +398,7 @@ class SettingsResponse(BaseModel):
     glm_mode: bool = False  # True if GLM API is configured via .env
     ollama_mode: bool = False  # True if Ollama API is configured via .env
     testing_agent_ratio: int = 1  # Regression testing agents (0-3)
+    playwright_headless: bool = True
 
 
 class ModelsResponse(BaseModel):
@@ -411,6 +412,7 @@ class SettingsUpdate(BaseModel):
     yolo_mode: bool | None = None
     model: str | None = None
     testing_agent_ratio: int | None = None  # 0-3
+    playwright_headless: bool | None = None
 
     @field_validator('model')
     @classmethod