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/internal/api/templates/day.html

@@ -15,7 +15,14 @@
       </select>
     </div>
     <input type="text" name="title" placeholder="Überschrift">
-    <textarea name="description" rows="4" placeholder="Beschreibung"></textarea>
+    <div class="editor-wrap">
+      <textarea name="description" rows="6" placeholder="Beschreibung — Markdown unterstützt&#10;Medien: per Drag & Drop oder Einfügen (Strg+V)"></textarea>
+      <div class="editor-bar">
+        <button type="button" class="media-picker">&#128206; Datei anhängen</button>
+        <span class="upload-status"></span>
+        <input type="file" class="media-file-input" multiple accept="image/*,video/*,audio/*" style="display:none">
+      </div>
+    </div>
     <div class="gps-row">
       <input type="number" name="lat" id="entry-lat" step="any" placeholder="Breite">
       <input type="number" name="lon" id="entry-lon" step="any" placeholder="Länge">
@@ -23,8 +30,6 @@
     </div>
     <small id="gps-status"></small>
     <input type="text" name="hashtags" placeholder="Hashtags (kommagetrennt)">
-    <input type="file" name="images" multiple accept="image/*" id="image-input">
-    <div id="image-preview" class="image-preview"></div>
     <button type="submit">Speichern</button>
   </form>
 
@@ -38,15 +43,21 @@
       <a href="/entries/{{.EntryID}}/edit" class="entry-edit">bearbeiten</a>
     </div>
     {{if .Title}}<div class="entry-title">{{.Title}}</div>{{end}}
-    {{if .Description}}<div class="entry-desc">{{.Description}}</div>{{end}}
+    {{if .Description}}<div class="entry-desc">{{markdown .Description}}</div>{{end}}
     {{if .Hashtags}}<div class="hashtags">{{range .Hashtags}}<span class="tag">#{{.}}</span> {{end}}</div>{{end}}
     {{if .Images}}
     <div class="entry-images">
       {{range .Images}}
+      {{if isVideo .MimeType}}
+      <video src="/uploads/{{.Filename}}" controls class="media-embed"></video>
+      {{else if isAudio .MimeType}}
+      <audio src="/uploads/{{.Filename}}" controls class="media-audio"></audio>
+      {{else}}
       <a href="/uploads/{{.Filename}}" target="_blank">
         <img src="/uploads/{{.Filename}}" alt="{{.OriginalName}}" class="thumb">
       </a>
       {{end}}
+      {{end}}
     </div>
     {{end}}
   </div>
@@ -96,6 +107,7 @@
 
 {{define "scripts"}}
 <script src="/static/day.js"></script>
+<script src="/static/editor.js"></script>
 {{end}}
 
 {{template "base" .}}