ci: refresh README screenshots on a hosted emulator

[vide]

What this does

PR 2 of the screenshot plan, on top of #381 (replaces #382, which GitHub closed when its base branch was deleted). Adds the Screenshots GitHub Actions workflow: run it by hand after a UI change and it boots a headless emulator on a hosted runner, captures the gallery from demo mode with the suite from #381, and opens a pull request on chore/refresh-screenshots with the changed images and README block. No PR is opened when nothing changed.

How it works

• workflow_dispatch with inputs screens (subset of spec ids), clock (override the pinned instant) and dry_run (artifacts only). One run at a time via a concurrency group.
• KVM enabled on the runner, API 35 google_apis x86_64 on the pixel_6 profile (1080×2400 at 420 dpi, the geometry the current gallery came from), software GPU, Europe/Madrid timezone, animations off. The AVD snapshot is cached between runs; the first run also sets hide_error_dialogs before the snapshot is saved.
• The capture is scripts/screenshots.sh, the same entry point as locally, called through .github/scripts/capture-screenshots.sh (the emulator-runner action executes each script line with /bin/sh, so the retry and the logcat dump live in a bash helper). The google_apis image is rootable, so the script pins device clock and demo clock together and a same-day re-run is pixel-identical.
• Every run uploads the suite's output directory as an artifact: the images, and for failed screens a hierarchy dump and a raw frame, plus logcat.txt when both attempts fail.
• peter-evans/create-pull-request opens the PR with the workflow token. That means CI does not run on the refresh PR (only images and README change) and the repository setting Allow GitHub Actions to create and approve pull requests must be on. I enabled it.

Found and fixed while validating on the branch

• The launcher raised an "isn't responding" dialog during the cold boot that produced the AVD snapshot. While such a dialog is up the app's window is absent from the accessibility tree, so every marker timed out. The suite now dismisses ANR and crash dialogs (by the system's button ids, locale-independent) before and during each wait.

Validation

Three validation runs on this branch (temporary push trigger):

Run	Outcome	Cause
1	failed in 2 min	bash syntax in a /bin/sh script line
2	failed in 28 min	launcher ANR dialog hid the app window from UiAutomator
3	green in 8m45s, 11/11 captured	compile ~5 min, capture 85 s, both clocks pinned

The images from run 3 are in its screenshots-output artifact; they are not committed here because the PR step is skipped on branch runs. The first workflow_dispatch after merge will open the refresh PR.

The temporary push trigger used for validation has been removed; only workflow_dispatch remains. First real dispatch after merge will exercise the PR-opening step, which the branch runs skipped by design.

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