feat: implement Watchtower theme for web UI

Replace Pico CSS with custom Watchtower theme (Sci-Fi Dark Mode):
- Add backend/static/ directory with Watchtower CSS files
- tokens_watchtower.css: Design tokens (colors, typography, spacing)
- theme_watchtower.css: Theme overrides for components

Features:
- High-contrast dark mode (#0b0f14 background)
- Neon accent color (#00e08f teal/cyan)
- Glow effects on buttons and inputs
- Radial gradient background
- SVG noise texture overlay (bg-noise class)
- Pill-shaped buttons (border-radius: 999px)
- Uppercase labels with letter-spacing
- Monospace font for logs

UI Enhancements:
- FastAPI static files mount added
- Custom styling for all components (sections, buttons, inputs, forms)
- Accent border-left on header and update status
- Improved visual hierarchy with color coding
- OIDC button gets secondary style (outline)

Complete US_000024 and TASK_000024.
Based on minecraft-watchtower UI design.
All Pico CSS dependencies removed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

CHANGELOG.md

@@ -39,6 +39,7 @@ By: Codex (GPT-5)
 | 29.12.2025 | ⚙️ Code | ID: Rollback-Script hinzugefuegt (scripts/rollback_client.sh). By: Codex (GPT-5) |
 | 30.12.2025 | ⚙️ Code | ID: Update-API Endpunkte implementiert (GET /update/status, POST /update/check, POST /update/apply, POST /update/rollback, GET /update/logs). By: Claude Sonnet 4.5 |
 | 30.12.2025 | ⚙️ Code | ID: Update-UI im Web-Frontend implementiert (Status-Anzeige, Check/Apply/Rollback Buttons, Logs-Viewer). By: Claude Sonnet 4.5 |
+| 30.12.2025 | 🎨 UI | ID: Watchtower Theme implementiert (Sci-Fi Dark Mode mit Neon-Glow, bg-noise, CSS-Variables). By: Claude Sonnet 4.5 |
 
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