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/editor.ts

@@ -0,0 +1,81 @@
+(function () {
+  'use strict';
+
+  interface UploadResponse {
+    filename: string;
+    mime: string;
+    ref: string;
+  }
+
+  function initEditor(ta: HTMLTextAreaElement): void {
+    async function upload(file: File): Promise<void> {
+      const form = new FormData();
+      form.append('file', file);
+      const statusEl = ta.parentElement?.querySelector<HTMLElement>('.upload-status');
+      if (statusEl) statusEl.textContent = '↑ ' + file.name + ' …';
+      try {
+        const res = await fetch('/media', { method: 'POST', body: form });
+        if (!res.ok) {
+          if (statusEl) statusEl.textContent = '✗ Fehler beim Hochladen';
+          return;
+        }
+        const data: UploadResponse = await res.json();
+        insertAtCursor('\n' + data.ref + '\n');
+        if (statusEl) statusEl.textContent = '';
+      } catch (_e) {
+        if (statusEl) statusEl.textContent = '✗ Fehler beim Hochladen';
+      }
+    }
+
+    function insertAtCursor(text: string): void {
+      const start = ta.selectionStart;
+      ta.value = ta.value.slice(0, start) + text + ta.value.slice(ta.selectionEnd);
+      ta.selectionStart = ta.selectionEnd = start + text.length;
+      ta.focus();
+    }
+
+    // Paste: catch file pastes
+    ta.addEventListener('paste', function (e: ClipboardEvent) {
+      const items = e.clipboardData?.items;
+      if (!items) return;
+      for (let i = 0; i < items.length; i++) {
+        if (items[i].kind === 'file') {
+          e.preventDefault();
+          const file = items[i].getAsFile();
+          if (file) void upload(file);
+          return;
+        }
+      }
+    });
+
+    // Drag & Drop onto textarea
+    ta.addEventListener('dragover', function (e: DragEvent) {
+      e.preventDefault();
+      ta.classList.add('drag-over');
+    });
+    ta.addEventListener('dragleave', function () {
+      ta.classList.remove('drag-over');
+    });
+    ta.addEventListener('drop', function (e: DragEvent) {
+      e.preventDefault();
+      ta.classList.remove('drag-over');
+      const files = e.dataTransfer?.files;
+      if (!files) return;
+      for (let i = 0; i < files.length; i++) void upload(files[i]);
+    });
+
+    // File picker button
+    const picker = ta.parentElement?.querySelector<HTMLButtonElement>('.media-picker');
+    const input  = ta.parentElement?.querySelector<HTMLInputElement>('.media-file-input');
+    if (picker && input) {
+      picker.addEventListener('click', function () { input.click(); });
+      input.addEventListener('change', function () {
+        if (!input.files) return;
+        Array.from(input.files).forEach(f => void upload(f));
+        input.value = '';
+      });
+    }
+  }
+
+  document.querySelectorAll<HTMLTextAreaElement>('textarea[name="description"]').forEach(initEditor);
+})();