feat: replace Watchtower theme with OIDC professional design

- Remove Watchtower theme CSS files (tokens_watchtower.css, theme_watchtower.css)
- Add OIDC styles.css (professional IT asset management design)
- Add dark mode toggle button (moon/sun icon)
- Remove custom CSS in favor of OIDC styles.css
- Add dark mode persistence with localStorage
- Remove bg-noise div (Watchtower-specific)
- Wrap content in .container div for OIDC layout

Benefits:
- Professional gradient header
- Light/Dark mode toggle
- Better color palette for readability
- Consistent with homelab-service-oidc design

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

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

CHANGELOG.md

@@ -41,6 +41,7 @@ By: Codex (GPT-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 |
 | 30.12.2025 | 🎨 UI | ID: Web-UI modernisiert (Lucide Icons, Metrics Dashboard, User Table mit Badges, Toast Notifications, Loading States, Action Modals). By: Claude Sonnet 4.5 |
+| 30.12.2025 | 🎨 UI | ID: Watchtower Theme ersetzt durch OIDC Theme (Professional Design mit Dark Mode Toggle, Light/Dark Theme). By: Claude Sonnet 4.5 |
 
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