Add HA screensaver flow, auth hardening, and tablet recovery fixes

[loopy321]

Summary

This PR improves the companion app for dedicated Home Assistant tablet use.

It adds reliable HA-driven screensaver navigation, preserves the original local screen-off fallback, hardens long-lived WebView auth behavior, improves crash/boot/display recovery, and fixes fullscreen/dashboard rendering issues.

Included changes

• Add idle-driven HA screensaver navigation with ui_idle reporting
• Preserve original local screen-off behavior when no screensaver dashboard path is configured
• Fix screensaver re-entry after temporary dashboard navigations and revert flows
• Fix partial settings updates so screen_timeout changes do not accidentally disable screensaver behavior
• Preserve external_auth on HA path navigation
• Add SPA navigation with fallback to full page load when HA returns a same-route/no-op result
• Restore correct startup routing to /view-assist/clock when no HA dashboard is configured
• Harden HA token expiry and refresh handling for long-lived WebView sessions
• Improve display/app recovery after boot, app loss, and foreground loss scenarios
• Fix fullscreen/window inset issues that caused top-bar/shifted rendering
• Fix explicit screen-off actions so they physically turn off the display without immediate wake-up
• Apply a final cleanup pass to remove dead code and reduce duplication

Validation

Validated on the Lenovo tablet with HA log checks:

• build/install succeeds
• app starts on the expected home route
• idle transitions into screensaver correctly
• temporary dashboard navigation with revert returns to clock and then re-enters screensaver
• blank screensaver path restores original local screen-off behavior
• explicit screen-off switch/action turns the display off
• boot/display recovery brings the app back visibly after restart scenarios
• observed runs no longer reproduced the prior HA auth failures

Notes

This branch was developed alongside matching vaca and view_assist custom component changes. Some behaviors in this app PR assume those integration-side updates are present.

Related changes

This PR is one part of a coordinated update across the companion app and Home Assistant integrations.

Related PRs:

• VACA integration: Add screensaver coordination and local fallback handling ViewAssist_Companion_App#203
• view_assist integration: Forward tablet navigate/revert actions for VACA devices view_assist_integration#4

These changes are designed to work together to improve:

• screensaver activation and recovery
• dashboard navigation and revert behavior
• WebView authentication stability
• fallback to local screen-off behavior when no screensaver dashboard is configured

Recommended rollout: merge and deploy all three together.