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>

backend/templates/index.html

@@ -4,14 +4,33 @@
   <meta charset="UTF-8" />
   <meta name="viewport" content="width=device-width, initial-scale=1.0" />
   <title>Safe Kiddo Control</title>
-  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/@picocss/pico@2/css/pico.min.css" />
+  <link rel="stylesheet" href="/static/tokens_watchtower.css" />
+  <link rel="stylesheet" href="/static/theme_watchtower.css" />
   <style>
-    body { max-width: 960px; margin: auto; padding: 1.5rem; }
-    .log { white-space: pre-line; }
+    body { max-width: 1200px; margin: auto; padding: 2rem; }
+    .log { white-space: pre-line; font-family: var(--font-mono); background: rgba(10, 14, 20, 0.4); padding: 1rem; border-radius: var(--radius-sm); }
     form { margin-bottom: 1rem; }
+    header { background: var(--color-surface); padding: 2rem; border-radius: var(--radius-md); margin-bottom: 2rem; border: 1px solid var(--color-border); border-left: 4px solid var(--color-accent); }
+    header h1 { color: var(--color-accent); margin: 0 0 0.5rem 0; font-size: 2rem; text-transform: uppercase; letter-spacing: 0.05em; }
+    header p { color: var(--color-text-muted); margin: 0; }
+    section { background: var(--color-surface); padding: 1.5rem; border-radius: var(--radius-md); margin-bottom: 1.5rem; border: 1px solid var(--color-border); }
+    h3 { color: var(--color-accent); text-transform: uppercase; letter-spacing: 0.1em; font-size: 0.875rem; margin-bottom: 1rem; border-bottom: 1px solid var(--color-border); padding-bottom: 0.5rem; }
+    button { padding: 0.75rem 1.5rem; background: var(--color-accent); color: #081015; border: none; border-radius: 999px; cursor: pointer; font-weight: 600; text-transform: uppercase; letter-spacing: 0.05em; transition: all 0.2s; }
+    button:hover { box-shadow: 0 0 20px var(--wt-glow); transform: translateY(-2px); }
+    button:disabled { opacity: 0.5; cursor: not-allowed; box-shadow: none; }
+    button#oidcLogin { background: transparent; border: 1px solid var(--color-accent); color: var(--color-accent); }
+    button#oidcLogin:hover { background: rgba(0, 224, 143, 0.1); }
+    input, select { background: rgba(10, 14, 20, 0.6); border: 1px solid rgba(255, 255, 255, 0.15); color: var(--color-text); padding: 0.75rem; border-radius: var(--radius-sm); width: 100%; }
+    input:focus, select:focus { border-color: var(--color-accent); box-shadow: 0 0 15px var(--wt-glow); outline: none; }
+    .grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(200px, 1fr)); gap: 1rem; }
+    label { display: block; margin-bottom: 0.5rem; color: var(--color-text-muted); font-size: 0.875rem; text-transform: uppercase; letter-spacing: 0.05em; }
+    details { margin-top: 1rem; background: rgba(10, 14, 20, 0.3); padding: 1rem; border-radius: var(--radius-sm); border: 1px solid rgba(255, 255, 255, 0.05); }
+    summary { cursor: pointer; color: var(--color-accent); font-weight: 600; user-select: none; }
+    #updateStatus { background: rgba(10, 14, 20, 0.6); border: 1px solid var(--color-border); border-left: 3px solid var(--color-accent); }
   </style>
 </head>
 <body>
+  <div class="bg-noise"></div>
   <header>
     <h1>Safe Kiddo Control</h1>
     <p>Steuere Nutzerkonten über die lokale API. Stelle sicher, dass der API-Token gesetzt ist.</p>