Group switch tile channels under device headings

[mot-yelraf]

Summary

When switch devices share a location, keep the first device ID, settings gear, and location in the tile title. Group each additional device's channels beneath a centered, full-width heading with the same font, settings gear, and location. Remove repeated device IDs from channel labels and move device settings access into the headings.

Update the user guide and version to v0.26.253.8.

Verification

• npm run validate:pr: all 25 Chromium browser checks passed on macOS with Python 3.13.9.
• Rendered a two-device Greenhouse fixture and verified channel ordering, the full-width divider, matching computed heading fonts, and location text. Inspected the Chromium screenshot.
• git diff --check: passed.
• User verified the UI.

Contributor checklist

• This pull request is focused on one logical change.
• I added or updated tests for behavior changes. Existing browser checks and a temporary rendered fixture verified this presentation-only change; no permanent tests added.
• 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 compile (3.11 and 3.13) and regression tests 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

Local verification used isolated fixtures without a live MQTT broker, GPIO hardware, or onboarding. No controller, MQTT, or persistence behavior changed.