Add APNG Studio canvas extension (#2272)

* Add APNG Studio canvas extension

APNG Studio is an interactive GitHub Copilot canvas extension for building
Animated PNG (APNG) files from frames: draw or upload frames, tune per-frame
timing and compositing, preview live, send the result to a phone by QR, and
export an animated .png.

Adds extensions/apng-studio/ with the required .github/plugin/plugin.json and
assets/preview.png, and regenerates .github/plugin/marketplace.json.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address review feedback for APNG Studio

Security and robustness (extension.mjs):
- Reject "." and ".." project ids so canvas/action input cannot resolve
  outside artifacts/.
- Bound request bodies (1 MiB JSON, 40 MiB frame upload); return 413 when
  exceeded.
- Reject malformed JSON with 400 instead of coercing to {}, so a truncated
  body cannot trigger destructive routes such as /frames/clear.
- Scope shares per project so one canvas cannot rotate or stop another's
  share; the LAN server is shared and torn down once no shares remain.
- get_state treats hiddenFirst as active only with >=2 frames, matching the
  encoder, so a single-frame project reports the correct duration.

Accessibility (web/):
- Expose Pen/Eraser state with aria-pressed and keep it in sync in setTool.
- Mark the toast as an aria-live status region.

Packaging:
- Add extensions/apng-studio/.gitignore with artifacts/ so the documented
  runtime-data exclusion holds for this extension.
- Update the README install section to reference the committed awesome-copilot
  extension path.

Regenerated .github/plugin/marketplace.json.

* Use "GitHub Copilot app" wording

Update the plugin description (and regenerated marketplace entry) to refer to
the host as the GitHub Copilot app.

* Address deeper review for APNG Studio

Security:
- Require a per-server access token on every loopback data/mutation request
  (minted per canvas server, carried in the iframe URL, attached to every
  renderer request). Static assets stay public. Blocks a local process or
  cross-origin page from reading state or driving mutations.

Concurrency and durability:
- Serialize the load-mutate-save cycle per project; dedupe concurrent first
  loads; write project.json via temp file + rename.

Lifecycle:
- Track SSE clients per instance and end them before server.close().
- Stop a project's LAN share only when no other panel references it.

Correctness:
- Report exact numerator/denominator total duration; handle pointercancel.

QR and accessibility:
- Place QR version bits least-significant-bit first (versions 7-10).
- Associate delay/fps/dispose/blend labels with their inputs via for=.

* Address deeper concurrency and validation review for APNG Studio

- Run assemble() under the project lock; add_color_frame renders and appends
  within one lock.
- Validate uploaded frames (PNG chunk scan, size + dimension checks) before
  writing; first frame stores real dimensions; CanvasError maps to 400.
- Validate move delta; clear frames in memory before deleting files.
- Route the open-handler rename through applySettings.
- Deduplicate LAN share startup; clean up if the canvas closes mid-start.
- End SSE streams before server.close(); skip dead streams in broadcast.
- Exact fractional duration; drawing surface stays >= 1px; refresh state before
  re-seeding "start from last frame".

* Address review: PNG format validation, share bind, limits, a11y

- Validate uploaded frames are 8-bit RGBA non-interlaced PNGs (standard
  compression/filter); reject formats the codec can't encode.
- Cap projects at 600 frames (add + duplicate).
- Bind the LAN share server to the private address, not 0.0.0.0; require a
  private address and rebind when it changes while idle; build the URL from the
  bound address.
- Align width/height inputs and clampSize to the 2048 maximum.
- Size the drawing surface by width + aspect ratio for narrow panels.
- set_frame with an unknown frameId returns a validation error.

* Address review: encoded-byte budget, timing modes, id keys, load errors

- 256 MiB aggregate encoded-byte budget (add + duplicate); per-frame size
  tracked and backfilled on load, so a frame count alone no longer bounds
  assembly memory.
- Frame timing is one exclusive mode (delayMs | fps | delayNum/delayDen);
  combinations are rejected instead of producing hybrid delays.
- Collision-resistant project storage keys: safe ids are used verbatim (existing
  projects still load) and only unsafe ids get a hash suffix, so "foo/bar" and
  "foo?bar" can't share a directory.
- Load only treats a missing file as a new project; other read/parse errors
  surface instead of silently overwriting real data.
- Share server binds to a real LAN interface (skips virtual/VPN/container,
  prefers physical NICs and common ranges).
- Generated export names get millisecond + random suffix to avoid same-second
  overwrite.
- PNG validator accepts the encoder's Uint8Array so agent solid-color frames
  aren't rejected.

* Address review: crash-safe frame writes, server startup, APNG first frame

- Write the PNG before mutating project metadata on add/duplicate, and delete
  after persisting, so an interrupted disk op can't dangle a reference or
  advance unsaved state.
- Evict a project from the in-memory cache if its save fails, so a transient
  write error can't desync the live session from disk.
- Reject on the loopback server's listen error and drop a half-initialized
  instance so startup failures clean up and can retry.
- Normalize APNG dispose_op PREVIOUS to BACKGROUND on the first animated frame
  (spec requirement).
- Release decoded ImageBitmaps after use so a large upload batch doesn't retain
  native image memory.
- Surface otherwise-silent async UI failures via a toast; check the state
  response is ok before parsing it.

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

extensions/apng-studio/.github/plugin/plugin.json

@@ -0,0 +1,19 @@
+{
+  "name": "apng-studio",
+  "description": "Interactive GitHub Copilot app canvas extension for building Animated PNG (APNG) files from frames. Draw or upload frames, tune per-frame timing and compositing, preview live, send the result to your phone by QR, and export an animated .png.",
+  "version": "1.0.0",
+  "author": {
+    "name": "Andrea Griffiths",
+    "url": "https://github.com/AndreaGriffiths11"
+  },
+  "keywords": [
+    "animated-png",
+    "apng",
+    "copilot-canvas",
+    "frame-animation",
+    "image-export",
+    "interactive-canvas"
+  ],
+  "logo": "assets/preview.png",
+  "extensions": "."
+}