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/workspace/ImageBlock.tsx

@@ -0,0 +1,116 @@
+import React, { useEffect, useState } from 'react';
+import { createPortal } from 'react-dom';
+import { ImageOff, Loader2, X } from 'lucide-react';
+import { mediaObjectURL, parseMediaContent } from '../../lib/media';
+
+interface ImageBlockProps {
+  content: string;
+  canEdit: boolean;
+  onCaptionChange: (caption: string) => void;
+}
+
+// Image block: the picture (click = full screen) and an optional caption below it.
+export const ImageBlock: React.FC<ImageBlockProps> = ({ content, canEdit, onCaptionChange }) => {
+  const ref = parseMediaContent(content);
+  const [src, setSrc] = useState<string | null>(null);
+  const [failed, setFailed] = useState(false);
+  const [zoomed, setZoomed] = useState(false);
+  const [caption, setCaption] = useState(ref?.caption || '');
+
+  useEffect(() => {
+    setCaption(ref?.caption || '');
+  }, [ref?.caption]);
+
+  const mediaId = ref?.id;
+  useEffect(() => {
+    if (!mediaId) return;
+    let alive = true;
+    setFailed(false);
+    mediaObjectURL(mediaId)
+      .then((u) => alive && setSrc(u))
+      .catch(() => alive && setFailed(true));
+    return () => {
+      alive = false;
+    };
+  }, [mediaId]);
+
+  useEffect(() => {
+    if (!zoomed) return;
+    const onKey = (e: KeyboardEvent) => e.key === 'Escape' && setZoomed(false);
+    window.addEventListener('keydown', onKey);
+    return () => window.removeEventListener('keydown', onKey);
+  }, [zoomed]);
+
+  if (!ref) {
+    return <div className="text-xs text-status-err py-1">Bild-Verweis ist beschädigt: {content}</div>;
+  }
+
+  const commitCaption = () => {
+    if (caption.trim() !== ref.caption) onCaptionChange(caption.trim());
+  };
+
+  return (
+    <figure className="my-1 space-y-1">
+      {failed ? (
+        <div className="flex items-center gap-2 h-24 px-3 rounded-card border border-border bg-panel text-xs text-muted">
+          <ImageOff className="w-4 h-4" /> Bild nicht verfügbar (offline oder gelöscht)
+        </div>
+      ) : src ? (
+        <button
+          type="button"
+          onClick={() => setZoomed(true)}
+          title="Vergrößern"
+          className="block max-w-full rounded-card overflow-hidden border border-border bg-panel cursor-zoom-in"
+        >
+          <img src={src} alt={ref.caption} className="block max-w-full max-h-[28rem] object-contain" />
+        </button>
+      ) : (
+        <div className="flex items-center justify-center w-48 h-32 rounded-card border border-border bg-panel">
+          <Loader2 className="w-4 h-4 animate-spin text-muted" />
+        </div>
+      )}
+
+      {canEdit ? (
+        <input
+          type="text"
+          value={caption}
+          onChange={(e) => setCaption(e.target.value)}
+          onBlur={commitCaption}
+          onKeyDown={(e) => {
+            if (e.key === 'Enter') {
+              e.preventDefault();
+              (e.target as HTMLInputElement).blur();
+            }
+          }}
+          placeholder="Beschriftung hinzufügen…"
+          className="w-full bg-transparent border-0 p-0 text-xs text-muted placeholder-faint focus:outline-none focus:ring-0"
+        />
+      ) : (
+        ref.caption && <figcaption className="text-xs text-muted">{ref.caption}</figcaption>
+      )}
+
+      {zoomed &&
+        src &&
+        createPortal(
+          <div
+            role="dialog"
+            aria-label={ref.caption || 'Bild'}
+            onClick={() => setZoomed(false)}
+            className="fixed inset-0 z-[100] bg-black/85 flex flex-col items-center justify-center p-4 cursor-zoom-out"
+          >
+            <button
+              type="button"
+              onClick={() => setZoomed(false)}
+              title="Schließen (Esc)"
+              className="absolute top-[calc(0.75rem+env(safe-area-inset-top))] right-3 w-11 h-11 flex items-center justify-center rounded-full bg-black/50 text-white"
+            >
+              <X className="w-5 h-5" />
+            </button>
+            <img src={src} alt={ref.caption} className="max-w-full max-h-[85vh] object-contain" />
+            {ref.caption && <p className="mt-3 text-sm text-white/90 text-center max-w-2xl">{ref.caption}</p>}
+          </div>,
+          document.body
+        )}
+    </figure>
+  );
+};