polish(share): picker avatars + target name + pre-warm video poster

Follow-ups from testing the share flow (which now works end-to-end):
- Send-to picker showed only initials — now shows the real profile photo when the
  chat has one (matching the sidebar/forward avatars), coloured initials otherwise.
- During send it only said "Uploading 60%" with no idea WHO to — now the header and
  progress name the target ("Sending to Manasa Rapolu · 60%").
- After sending, a video bubble sat blank (just a timestamp) for a second or two
  while the poster generated on first view. media.js now warms the poster thumbnail
  at UPLOAD (temp-then-rename), and the on-demand /thumbs handler also writes via a
  temp, so the two can't serve a half-written JPEG. The bubble shows its poster
  right away.

Web + server only — live on deploy. Does NOT address the share extension failing to
auto-open the app (an iOS limitation, handled next in the native build).

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

server/static.js

@@ -223,8 +223,12 @@ function handleGet(req, res) {
       const rs = fs.createReadStream(thumb); rs.on('error', () => { try { res.destroy(); } catch (e2) {} }); rs.pipe(res);
     });
     if (fs.existsSync(thumb)) return send();
-    return require('child_process').execFile('ffmpeg', ['-y', '-ss', '0.5', '-i', src, '-frames:v', '1', '-vf', 'scale=480:-2', '-q:v', '4', thumb], { timeout: 15000 }, (err) => {
-      if (err) { try { fs.unlinkSync(thumb); } catch (_) {} return json(res, 404, { error: 'thumbnail unavailable' }); }
+    // Write to a temp then rename — media.js may pre-generate the same poster at upload, and two writers
+    // to the same path could otherwise serve a half-written JPEG.
+    const tmp = thumb + '.req.part';
+    return require('child_process').execFile('ffmpeg', ['-y', '-ss', '0.5', '-i', src, '-frames:v', '1', '-vf', 'scale=480:-2', '-q:v', '4', tmp], { timeout: 15000 }, (err) => {
+      if (err) { try { fs.unlinkSync(tmp); } catch (_) {} return json(res, 404, { error: 'thumbnail unavailable' }); }
+      try { if (!fs.existsSync(thumb)) fs.renameSync(tmp, thumb); else fs.unlinkSync(tmp); } catch (_) {}
       send();
     });
   }