feat(frontend): add dark/light theme toggle with system detection

- Add ThemeToggle component (Auto / Dark / Light) to all views
- Store preference per browser and detect system preference live
  via prefers-color-scheme
- Apply theme before first render in index.html to prevent flash
- Extend CSS with light theme variables and refactor glass styles

frontend/index.html

@@ -5,6 +5,20 @@
     <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <meta name="theme-color" content="#05070c" />
+    <script>
+      // Design vor dem ersten Render anwenden (verhindert Flash beim Laden)
+      ;(function () {
+        try {
+          var m = localStorage.getItem('wannpassts_theme')
+          var dark =
+            m === 'dark' ||
+            ((m === null || m === 'auto') && window.matchMedia('(prefers-color-scheme: dark)').matches)
+          var t = dark ? 'dark' : 'light'
+          document.documentElement.dataset.theme = t
+          document.documentElement.style.colorScheme = t
+        } catch (e) {}
+      })()
+    </script>
     <meta
       name="description"
       content="WannPassts – Deine freien Zeiten, geteilt per Link. Verbinde Google-, iCloud- und andere Kalender und nimm Buchungsanfragen an, ohne deine Termine preiszugeben."