Refine dashboard branding and date header

[mot-yelraf]

Summary

Move the footer overview image into the dashboard header in place of the text title. Display it at 50% of its former width (261px), with responsive sizing and rounded transparent corners, between separate vertically centered graph and settings pills. Abbreviate the weekday in the date/time display.

Remove footer-pinning behavior from sensor ordering, update header layout coverage and the user guide, and bump the version to v0.26.251.13.

Verification

• npm run validate:pr: all 9 Chromium checks passed on macOS, Python 3.13.9, using the isolated host fixture.
• python3 -m pytest testApparatus/test_sai_html_gauge_config.py testApparatus/test_dashboard_button_labels.py -q: 34 checks passed during implementation; the 32 gauge configuration checks also passed after the size adjustment.
• Verified desktop/mobile header screenshots, weekday abbreviation, and git diff --check.
• User verified the final appearance.

Contributor checklist

• This pull request is focused on one logical change.
• I added or updated tests for behavior changes.
• I updated the canonical documentation when behavior or configuration changed.
• I did not commit credentials, private runtime configuration, databases, or logs.
• I preserved compatibility-sensitive MQTT, settings, switch identity, and persistence behavior, or documented an intentional break.

Maintainer verification

• Required remote checks pass (Python 3.11/3.13 compile and full regression suite with coverage).
• npm run validate:pr passes on a trusted host when the change can affect rendered UI behavior.
• Hardware-, broker-, onboarding-, and platform-specific behavior not covered remotely is recorded below.

Residual risk or unverified areas

Validation used the macOS host fixture without live MQTT or direct sensor hardware. Hardware, onboarding, and other operating systems were not exercised; this change concerns dashboard presentation and sensor group placement.