Add TypeScript migration, image resizing, media upload UX, and multimedia support

- Migrate static JS to TypeScript (static-ts/ → compiled to internal/api/static/)
- Add image resizing on upload: JPEG/PNG/WebP scaled to max 1920px at quality 80
- Extract shared upload logic into upload.go (saveUpload, saveResizedImage, saveResizedWebP)
- Add POST /media endpoint for drag-drop/paste media uploads with markdown ref return
- Add background music player with video/audio coordination (autoplay.ts)
- Add global nav, public feed, hashtags, visibility, Markdown rendering for entries
- Add Dockerfile stage for TypeScript compilation (static-ts-builder)
- Add goldmark, disintegration/imaging, golang.org/x/image dependencies

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/static-ts/day.ts

@@ -0,0 +1,31 @@
+// GPS button
+document.getElementById('btn-gps')?.addEventListener('click', function () {
+  const status = document.getElementById('gps-status') as HTMLElement;
+  if (!navigator.geolocation) {
+    status.textContent = '// GPS nicht verfügbar';
+    return;
+  }
+  status.textContent = '// Standort wird ermittelt...';
+  navigator.geolocation.getCurrentPosition(
+    function (pos: GeolocationPosition) {
+      (document.getElementById('entry-lat') as HTMLInputElement).value = pos.coords.latitude.toFixed(6);
+      (document.getElementById('entry-lon') as HTMLInputElement).value = pos.coords.longitude.toFixed(6);
+      status.textContent = '// Standort gesetzt (' + pos.coords.accuracy.toFixed(0) + ' m Genauigkeit)';
+    },
+    function (err: GeolocationPositionError) {
+      status.textContent = '// Fehler: ' + err.message;
+    },
+    { enableHighAccuracy: true, timeout: 10000 }
+  );
+});
+
+// Set current time as default
+(function () {
+  const input = document.getElementById('entry-time') as HTMLInputElement | null;
+  if (input && !input.value) {
+    const now = new Date();
+    const hh = String(now.getHours()).padStart(2, '0');
+    const mm = String(now.getMinutes()).padStart(2, '0');
+    input.value = hh + ':' + mm;
+  }
+})();