perf(renderer): keep recent-uploads panel append-only past the cap (kill ~80ms per-completion freeze)

The recent-uploads panel had a cheap append-only fast path, but it was gated on
`rows.length > _recentLastRenderedLen`. maybeAddSessionFile caps sessionFilesData
by push-then-slice (2000 -> 2001 -> sliced back to 2000), so once a session
produces more than SESSION_FILES_CAP rows the length is pinned at the cap and the
gate is false forever. Every subsequent completion then fell through to the full
`tbody.innerHTML = rows.map(...).join('')` rebuild of all ~2000 rows.

The cap is per (link x file x hoster), so with 4-5 selected hosters the 2000 cap
is hit at only ~400-500 distinct files — very reachable in a long folder-monitor
session. Profiled in Chromium (same Blink engine as Electron, table-layout:fixed):
the full 2000-row rebuild costs ~80ms and ran on EVERY completion past the cap — a
repeating ~80ms main-thread freeze. That is the "fine on a fresh start, gets laggy
after many uploads while CPU (~40%) and RAM (~6GB, stable) stay normal" symptom: a
render-thread stall, not CPU saturation or a memory leak.

Fix: track newly-pushed rows in _recentPendingAppends (incremented in
maybeAddSessionFile, consumed every render) and gate the fast path on
`pendingAppends > 0` instead of length growth, so it survives the cap. Prepend the
new rows, then evict the same overflow count from the DOM bottom (oldest rows,
which is where the date-desc view places the front-of-array entries the cap slices
off). DOM work is O(added) again. The fast path is gated behind an explicit
`appendOnly` flag passed only by scheduleRecentRender's rAF, so selection / delete
/ clear / sort / batch-done renders stay full rebuilds and cannot wrong-evict or
double-prepend.

Verified in Blink over a simulated 5000-completion session (3/frame, far past the
cap): per-frame render 80ms -> median 7.4ms (>10x), and the DOM stays exactly equal
to the data (cap held at 2000, newest-on-top, oldest evicted, zero duplicates).
397/397 tests pass, eslint clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

renderer/app.js

@@ -1177,7 +1177,7 @@ let _recentRenderQueued = false;
 function scheduleRecentRender() {
   if (_recentRenderQueued) return;
   _recentRenderQueued = true;
-  requestAnimationFrame(() => { _recentRenderQueued = false; renderRecentUploadsPanel(); });
+  requestAnimationFrame(() => { _recentRenderQueued = false; renderRecentUploadsPanel(true); });
 }
 
 // Toggle the .selected class on existing rows without rebuilding the table.
@@ -2768,6 +2768,7 @@ function maybeAddSessionFile(job) {
       });
       _recentDataVersion++;
       _sessionDoneCount++;
+      _recentPendingAppends++;
       // Drop oldest entries past the cap to keep render cost bounded.
       // Without this, sessionFilesData grows unbounded across the session
       // and every renderRecentUploadsPanel call becomes a megabyte-sized
@@ -4672,10 +4673,13 @@ function _buildRecentRowHtml(row) {
 // accumulating new uploads (the default case: sort=date desc, rows only grow).
 let _recentLastRenderedSig = '';
 let _recentLastRenderedLen = 0;
+let _recentPendingAppends = 0;
 
-function renderRecentUploadsPanel() {
+function renderRecentUploadsPanel(appendOnly = false) {
   const tbody = document.getElementById('recentFilesBody');
   if (!tbody) return;
+  const pendingAppends = _recentPendingAppends;
+  _recentPendingAppends = 0;
   if (!sessionFilesData.length) {
     tbody.innerHTML = '<tr><td colspan="4" class="empty-state">Noch keine Uploads in dieser Session.</td></tr>';
     _recentLastRenderedSig = '';
@@ -4685,9 +4689,10 @@ function renderRecentUploadsPanel() {
 
   const rows = sortRecentFiles(sessionFilesData);
   const sig = `${recentSortState.key}|${recentSortState.direction}`;
-  const dateDescAppendOnly = sig === 'date|desc'
+  const dateDescAppendOnly = appendOnly
+    && pendingAppends > 0
+    && sig === 'date|desc'
     && _recentLastRenderedSig === sig
-    && rows.length > _recentLastRenderedLen
     && tbody.querySelectorAll('.recent-file-row').length === _recentLastRenderedLen;
 
   const wrap = tbody.closest('.recent-files-table-wrap');
@@ -4695,10 +4700,17 @@ function renderRecentUploadsPanel() {
 
   let wasAppendOnly = false;
   if (dateDescAppendOnly) {
-    const added = rows.length - _recentLastRenderedLen;
+    const added = Math.min(pendingAppends, rows.length);
     let html = '';
     for (let i = 0; i < added; i++) html += _buildRecentRowHtml(rows[i]);
     tbody.insertAdjacentHTML('afterbegin', html);
+    let evict = (_recentLastRenderedLen + added) - rows.length;
+    while (evict > 0) {
+      const last = tbody.lastElementChild;
+      if (!last) break;
+      last.remove();
+      evict--;
+    }
     wasAppendOnly = true;
   } else {
     tbody.innerHTML = rows.map(_buildRecentRowHtml).join('');