fix: audit src/js and add a frame-accurate export path

Two halves of one pass; TODO-FIXES.md lists every finding.

The audit fixed, among ~90 items: MP4 export dead-ending and locking the UI
behind a stuck "Downloading..." button; height keyframes storing the width;
shadow defaults and keyframes storing undefined because fabric's get() is not
a path getter; every letter animation writing to the last letter; keyframe
times drifting on each drag because data-time was layer-relative on expanded
rows; O(n^2 log n) playback, now indexed once per frame; and a save() that
rebuilt the record canvas on every edit, now debounced.

Exports also lost audio-layer sound outright: MediaRecorder records only the
first audio track, so every source now mixes through one AudioContext into one
destination.

The new half is src/js/render.js, an offline renderer. Real-time capture was
the root cause of dropped frames on heavy scenes and smeared video layers: a
bare currentTime assignment is async, so drawing straight after it captures the
previous frame. Each frame is now seeked, awaited on 'seeked', drawn, and
pushed through a VideoEncoder; audio is mixed in one OfflineAudioContext pass
and encoded to Opus. webm-writer2.js gained a second Opus track and lost three
bugs, including a MAX_CLUSTER_DURATION_MSEC of ~58 days that overflowed the
signed 16-bit block timecode past ~32s.

Real-time capture remains the fallback: the hand-rolled muxer's output is
decoded in a <video> element before being handed over, and any failure returns
null so record() falls back transparently.

test/webm-muxer.test.js parses the muxer output with an EBML reader in plain
Node. It caught the fixed-256-byte header overflow that inspection missed.

Not verified in a browser: node --check passes on every script and the muxer
test passes, but a running Chrome held the Playwright profile lock, so the
manual pass listed at the end of TODO-FIXES.md is still outstanding.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/js/align.js

@@ -78,6 +78,20 @@ function initLines() {
   canvas.add(line_v);
 }
 
+// Hide the alignment guides after a canvas reload.
+// They can be missing entirely in projects saved before they existed, so
+// never dereference them directly.
+function hideGuides(inst) {
+  const h = inst.getItemById('line_h');
+  const v = inst.getItemById('line_v');
+  if (h) {
+    h.set({ opacity: 0 });
+  }
+  if (v) {
+    v.set({ opacity: 0 });
+  }
+}
+
 function alignControls(object, type) {
   if (type == 'align-top') {
     object.set(
@@ -122,8 +136,10 @@ function alignControls(object, type) {
 function alignObject() {
   const type = $(this).attr('id');
   const object = canvas.getActiveObject();
-  console.log(canvas.getActiveObject().type);
-  if (canvas.getActiveObject().type == 'activeSelection') {
+  if (!object) {
+    return;
+  }
+  if (object.type == 'activeSelection') {
     const tempselection = canvas.getActiveObject();
     canvas.discardActiveObject();
     tempselection._objects.forEach(function (object) {
@@ -157,6 +173,8 @@ function alignObject() {
     );
     newKeyframe('top', object, currenttime, object.get('top'), true);
   }
+  canvas.renderAll();
+  save();
 }
 $(document).on('click', '.align', alignObject);
 
@@ -240,7 +258,12 @@ function centerLines(e) {
           canvas.renderAll();
         }
       }
-      if (obj != e.target && obj != line_h && obj != line_v) {
+      if (
+        obj != e.target &&
+        obj != line_h &&
+        obj != line_v &&
+        obj.visible !== false
+      ) {
         if (
           obj.get('id') == 'center_h' ||
           obj.get('id') == 'center_v'