feat: Twitter-style UI theme + Playwright optimization + documentation

UI Changes:
- Replace neobrutalism with clean Twitter/Supabase-style design
- Remove all shadows, use thin borders (1px)
- Single accent color (Twitter blue) for all status indicators
- Rounded corners (1.3rem base)
- Fix dark mode contrast and visibility
- Make KanbanColumn themeable via CSS classes

Backend Changes:
- Default Playwright browser changed to Firefox (lower CPU)
- Default Playwright mode changed to headless (saves resources)
- Add PLAYWRIGHT_BROWSER env var support

Documentation:
- Add CUSTOM_UPDATES.md with all customizations documented
- Update .env.example with new Playwright options

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

.env.example

@@ -1,12 +1,19 @@
 # Optional: N8N webhook for progress notifications
 # PROGRESS_N8N_WEBHOOK_URL=https://your-n8n-instance.com/webhook/...
 
-# Playwright Browser Mode
-# Controls whether Playwright runs Chrome in headless mode (no visible browser window).
-# - true: Browser runs in background, invisible (recommended for using PC while agent works)
+# Playwright Browser Configuration
+#
+# PLAYWRIGHT_BROWSER: Which browser to use for testing
+# - firefox: Lower CPU usage, recommended (default)
+# - chrome: Google Chrome
+# - webkit: Safari engine
+# - msedge: Microsoft Edge
+# PLAYWRIGHT_BROWSER=firefox
+#
+# PLAYWRIGHT_HEADLESS: Run browser without visible window
+# - true: Browser runs in background, saves CPU (default)
 # - false: Browser opens a visible window (useful for debugging)
-# Defaults to 'false' if not specified
-# PLAYWRIGHT_HEADLESS=false
+# PLAYWRIGHT_HEADLESS=true
 
 # GLM/Alternative API Configuration (Optional)
 # To use Zhipu AI's GLM models instead of Claude, uncomment and set these variables.