Update for stacked generated-image notifications and icon refresh.

Summary:
- Added a toast stack so every incoming image gets its own preview notification for 5 seconds, with multiple generated images stacking downward.
- Kept non-image files visible too: mixed batches now show image toasts plus one grouped toast for the other files.
- Regenerated all 50 file type icons individually with gpt-image-2, then recropped/exported each as a 128x128 UI PNG.
- Added agent-context guidance: generated images/assets intended for the user must be sent with `paf-send` so they appear in the terminal bubble previews.
- Deployed the updated HTML and assets live without restarting `ttyd` or `ttyd-attach`.

Test plan:
- `node` syntax check for the embedded `paf-transfer-script`
- `find`/`identify` check: 50 icon PNGs, all 128x128
- `bash -n configs/paf-send scripts/deploy.sh scripts/provision.sh`
- `git diff --check`
- `go test ./...`
- Live API test: copied a generated PNG into the active session outbox and verified `/outbox` plus `GET /download` returns `image/png`