feat(workspace): image blocks via /bild (M2)

- /bild (only in an empty bullet) turns the bullet into an inline picker:
  file, camera on phones, drag & drop, paste, or an https link the server
  copies. Esc/✕ restores the empty bullet; nothing is synced before the
  upload succeeded.
- Photos are shrunk to 2048 px and stored as WebP (JPEG fallback); the
  re-encoding drops EXIF data such as the location.
- Image block: loaded with the login as blob URL, click opens full
  screen, optional caption kept in the block content
  (![caption](media:id)).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/help/HelpView.tsx

@@ -159,6 +159,9 @@ export const HelpView: React.FC<HelpViewProps> = ({ onBack }) => {
           'code',
           'callout',
           'divider',
+          'bild',
+          'image',
+          'foto',
           'auto-formatting',
         ],
         content: (
@@ -192,6 +195,10 @@ export const HelpView: React.FC<HelpViewProps> = ({ onBack }) => {
                   <span className="text-accent font-bold">—</span>
                   <span>/divider (Trennlinie)</span>
                 </div>
+                <div className="p-1.5 bg-panel rounded border border-border flex items-center gap-1.5 text-main col-span-2">
+                  <span className="text-accent font-bold">🖼️</span>
+                  <span>/bild (Bild hochladen, fotografieren, einfügen oder per Link; nur im leeren Punkt)</span>
+                </div>
               </div>
             </div>
             <div className="pt-2 border-t border-border">