feat(workspace): link previews above bullets with a web address (M5)

- GET /link-preview?url= fetches the page on the server (https only,
  SSRF-guarded like webhooks), reads Open Graph/Twitter tags, title,
  description and icon from the head, loads picture and icon (no SVG) and
  keeps them in link_previews (migration 6) for 7 days; readers' browsers
  never contact the page. Off in the demo; 300 fetches per address in
  15 minutes.
- Compact card above the bullet (icon, site, title, one line, small
  picture); the duplicate link badge is left out.
- ✕ turns the address into [title](url), so hiding syncs with the note;
  "Link-Vorschauen anzeigen" under appearance switches all off per device.

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

frontend/src/lib/linkPreview.ts

@@ -0,0 +1,85 @@
+// Link previews in notes: a card above a bullet that contains a plain web address. The server fetches
+// the page (https only) and keeps title, description, picture and icon; the browser only talks to
+// MindSift. Hiding the card for one link turns the address into a named link [Titel](url), so the
+// choice syncs with the note; a global switch (this device) turns all previews off.
+import { useEffect, useState } from 'react';
+import { getToken } from './api';
+
+export interface LinkPreview {
+  key: string;
+  url: string;
+  title: string;
+  description: string;
+  siteName: string;
+  hasImage: boolean;
+  hasIcon: boolean;
+  ok: boolean;
+}
+
+// First plain https address in a text (not inside [text](url), not right after "(" or "<")
+const BARE_URL_RE = /(^|[\s])(https:\/\/[^\s<>()\]]+[^\s<>()\].,;:!?'"])/;
+
+export function firstBareUrl(content: string): string | null {
+  const m = BARE_URL_RE.exec(content || '');
+  return m ? m[2] : null;
+}
+
+/** Replaces the address by a named link, which shows no preview. */
+export function hidePreviewContent(content: string, url: string, title: string): string {
+  const label = (title || new URL(url).hostname).replace(/[[\]]/g, '').trim() || url;
+  return content.replace(url, `[${label}](${url})`);
+}
+
+const cache = new Map<string, Promise<LinkPreview | null>>();
+
+export function loadLinkPreview(url: string): Promise<LinkPreview | null> {
+  let p = cache.get(url);
+  if (!p) {
+    p = (async () => {
+      const token = getToken();
+      const res = await fetch(`/api/v1/link-preview?url=${encodeURIComponent(url)}`, {
+        headers: token ? { Authorization: `Bearer ${token}` } : {},
+      });
+      if (!res.ok) return null; // demo, http link, offline: just no card
+      const data = (await res.json()) as LinkPreview;
+      return data.ok ? data : null;
+    })().catch(() => null);
+    cache.set(url, p);
+  }
+  return p;
+}
+
+const SETTING_KEY = 'mindsift_link_previews';
+const SETTING_EVENT = 'mindsift:link-previews';
+
+export function linkPreviewsEnabled(): boolean {
+  try {
+    return localStorage.getItem(SETTING_KEY) !== 'off';
+  } catch {
+    return true;
+  }
+}
+
+export function setLinkPreviewsEnabled(on: boolean): void {
+  try {
+    localStorage.setItem(SETTING_KEY, on ? 'on' : 'off');
+  } catch {
+    // private mode: only for this page
+  }
+  window.dispatchEvent(new CustomEvent(SETTING_EVENT));
+}
+
+/** The global switch, kept in sync across all open notes and tabs. */
+export function useLinkPreviewsEnabled(): boolean {
+  const [on, setOn] = useState(linkPreviewsEnabled);
+  useEffect(() => {
+    const update = () => setOn(linkPreviewsEnabled());
+    window.addEventListener(SETTING_EVENT, update);
+    window.addEventListener('storage', update);
+    return () => {
+      window.removeEventListener(SETTING_EVENT, update);
+      window.removeEventListener('storage', update);
+    };
+  }, []);
+  return on;
+}