mobile: Enter=newline on mobile, hide preview zoom buttons; desktop: downloads to Downloads folder (batch133)

- Mobile: the Return key now inserts a newline instead of sending (send via the button, like
  every native chat app). Desktop keeps Enter=send / Shift+Enter=newline.
- Image preview: hide the on-screen +/- zoom buttons on mobile (pinch-to-zoom covers it).
- Desktop (Electron): a will-download handler saves straight to the OS Downloads folder with no
  "where to save?" dialog, de-duping the name if it exists. NOTE: desktop code only — NOT
  published to the update feed (needs an explicit desktop rebuild/publish).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

desktop/main.js

@@ -437,6 +437,19 @@ function configureSession() {
     if (sys && sys !== 'en-US' && (!avail.length || avail.includes(sys))) langs.push(sys);
     ses.setSpellCheckerLanguages(langs);
   } catch (_) {}
+  // Downloads go STRAIGHT to the OS Downloads folder — no "where do you want to save?" dialog. If a file of
+  // the same name already exists, suffix " (n)" so nothing is overwritten. (item.setSavePath suppresses the
+  // save dialog entirely.)
+  ses.on('will-download', (_e, item) => {
+    try {
+      const dir = app.getPath('downloads');
+      const name = item.getFilename() || 'download';
+      const ext = path.extname(name), base = path.basename(name, ext);
+      let target = path.join(dir, name), n = 1;
+      while (fs.existsSync(target)) target = path.join(dir, `${base} (${n++})${ext}`);
+      item.setSavePath(target);
+    } catch (_) {}
+  });
 }
 
 // Single-instance: a tray app must not spawn a second copy. If another launch happens, focus the