e2e: wait for dashboard QR content before final screenshot

[guysoft]

Problem

The final E2E screenshot captured the desktop wallpaper / logo (stddev ~0.20) instead of the welcome dashboard with the QR code (stddev ~0.31). Tests still reported ALL TESTS PASSED, so the regression was silent.

Root cause (deterministic, not flaky — reproduced twice with identical stddev 0.20752):

1. Tests run alphabetically, so test_translation_disabled.sh is the last test before screenshot.sh.
2. Its restore_url() writes the dashboard URL back to fullpageos.txt, killall chromium, then returns without waiting for the kiosk to relaunch. run_onepageos respawns chromium, but the relaunched kiosk first paints the desktop shell/wallpaper and only later loads the welcome iframe (the QR code).
3. screenshot.sh accepts a candidate as soon as dashboard_is_painted is true, but PAINT_STDDEV_THRESHOLD was 0.08 — and the shell/wallpaper-only state already scores 0.20752, so it passed on the very first candidate before the QR iframe painted.

Stddev measurements (cropped 1280x620+0+80, same as the hook):

• shell/logo only: 0.20752
• German test page: 0.123093
• full dashboard with QR: 0.312033
• old threshold: 0.08

Fix

testing/hooks/screenshot.sh

• Raise PAINT_STDDEV_THRESHOLD default 0.08 → 0.26 (above the wallpaper/shell baseline 0.20, below the full QR dashboard 0.31).
• Require two consecutive passing captures (PAINT_STABLE_PASSES=2) so the screenshot is taken only after the welcome iframe has rendered and settled, not on a mid-load transition. On timeout the hook still fails and saves screenshot-not-painted.png.

testing/tests/test_translation_disabled.sh

• restore_url() now waits for the dashboard kiosk process to relaunch on the original URL (--kiosk --app=http://localhost/FullPageDashboard) before returning, so screenshot.sh starts from a relaunched kiosk rather than the bare desktop.

Evidence

Verified against CustomPiOS PR #274 (pr-274-local image) + FullPageOS devel (7c9b93d), QEMU aarch64.

Before (e2e_results/pr274-devel-rerun-20260713/, unchanged devel):

• screenshot.png = desktop wallpaper / logo, stddev 0.20752
• accepted on first candidate: Display screenshot saved after dashboard paint (1x2s)
• TEST_RESULT=0, exit-code 0 (false pass)

After (e2e_results/pr274-devel-qrfix-20260713/, this branch):

• restore_url: Dashboard kiosk restored (pid: 8480) after 2x2s
• paint loop: stddev 0 → 0.312033 → Painted but waiting for stabilization (1/2) → 0.312033 → Display screenshot saved after dashboard paint (2 stable passes, 3x2s)
• screenshot.png = welcome dashboard with QR code + setup speech bubble, stddev 0.312033
• TEST_RESULT=0, exit-code 0, ALL TESTS PASSED

The fix is on the FullPageOS side only; no CustomPiOS changes.

Test plan

• Re-ran E2E on unchanged devel to confirm the wallpaper screenshot reproduces (not flaky)
• Rebuilt E2E container with the fix and re-ran; final screenshot shows QR dashboard (stddev 0.312033)
• Confirm CI on this PR reproduces the QR-dashboard screenshot