feat(pwa): add web share target for text and images with offline queue

frontend/public/sw.js

@@ -5,10 +5,12 @@
 // - Icons and manifest: from the cache, refreshed in the background.
 // - API requests are never touched; offline data comes from the app's own cache (lib/offline.ts).
 // Raise VERSION when this file changes in a way that needs fresh caches.
-const VERSION = 'v2';
+const VERSION = 'v3';
 const SHELL_CACHE = `mindsift-shell-${VERSION}`;
 const ASSET_CACHE = `mindsift-assets-${VERSION}`;
 const MAX_ASSETS = 80;
+const SHARE_DB_NAME = 'mindsift-share-target';
+const SHARE_STORE_NAME = 'shared_items';
 const SHELL_URLS = [
   '/',
   '/index.html',
@@ -80,8 +82,78 @@ async function staleWhileRevalidate(request) {
   return hit || refresh;
 }
 
+function openShareDB() {
+  return new Promise((resolve, reject) => {
+    const req = indexedDB.open(SHARE_DB_NAME, 1);
+    req.onupgradeneeded = () => {
+      const db = req.result;
+      if (!db.objectStoreNames.contains(SHARE_STORE_NAME)) {
+        db.createObjectStore(SHARE_STORE_NAME, { autoIncrement: true, keyPath: 'id' });
+      }
+    };
+    req.onsuccess = () => resolve(req.result);
+    req.onerror = () => reject(req.error);
+  });
+}
+
+async function savePendingShare(item) {
+  const db = await openShareDB();
+  return new Promise((resolve, reject) => {
+    const tx = db.transaction(SHARE_STORE_NAME, 'readwrite');
+    tx.objectStore(SHARE_STORE_NAME).add(item);
+    tx.oncomplete = () => {
+      db.close();
+      resolve();
+    };
+    tx.onerror = () => {
+      db.close();
+      reject(tx.error);
+    };
+    tx.onabort = () => {
+      db.close();
+      reject(tx.error);
+    };
+  });
+}
+
+async function handleShareTarget(event) {
+  try {
+    const formData = await event.request.formData();
+    const title = formData.get('title');
+    const text = formData.get('text');
+    const url = formData.get('url');
+    const media = formData.get('media');
+
+    let fileName = '';
+    let hasMedia = false;
+    if (media && typeof media === 'object') {
+      fileName = media.name || 'image';
+      hasMedia = true;
+    }
+
+    await savePendingShare({
+      title: typeof title === 'string' ? title.trim() : '',
+      text: typeof text === 'string' ? text.trim() : '',
+      url: typeof url === 'string' ? url.trim() : '',
+      fileName,
+      hasMedia,
+      createdAt: Date.now(),
+    });
+  } catch (err) {
+    console.error('[SW] Share-Target Fehler:', err);
+  }
+  return Response.redirect('/?shared=pending', 303);
+}
+
 self.addEventListener('fetch', (event) => {
   const { request } = event;
+  if (request.method === 'POST') {
+    const url = new URL(request.url);
+    if (url.origin === self.location.origin && url.pathname === '/share-target') {
+      event.respondWith(handleShareTarget(event));
+    }
+    return;
+  }
   if (request.method !== 'GET') return;
   const url = new URL(request.url);
   if (url.origin !== self.location.origin || url.pathname.startsWith('/api/')) return;