fix(settings): picking a theme switches Material You off so the pick applies

[ErikChevalier]

What & why

The theme dropdowns worked mechanically (the choice persisted and applied with Material You off), but Material You is on by default and overrides the named themes, so on most devices picking a theme appeared to do nothing. The only clue was the hint sentence below the second dropdown. Reported as "theme selection doesn't do anything".

Picking a named theme is an explicit statement of intent to use it, so the pick now switches dynamic color off in the same edit and takes effect immediately. Re-enabling Material You afterwards is respected and keeps the slot choice. The hint text still reads correctly for the remaining case (toggling Material You on after a pick), and no translated strings change.

Verification

• Unit tests: picking a light/dark theme turns dynamic color off and persists the slot; re-enabling Material You afterwards is respected.
• On the emulator (fresh install, Material You on, exactly the reported state): picked Rose Pine Dawn from the Light dropdown and it applied instantly with the toggle flipping off; Dark slot verified with Tokyo Night, Nord, and Gruvbox, each applying immediately with independent slot persistence across restart.
• ktlint and the JVM test suite are green.

Checklist

• Built on its own fix/ branch off main
• ./gradlew ktlintCheck lint test assembleDebug is green locally
• Added/updated tests (unit and/or instrumentation)
• No telemetry, trackers, or device identifiers added
• No Google scraping introduced
• Battery: no wake-lock held while idle
• OpenSpec change validates (openspec validate <name> --strict) and tasks are checked off — n/a (small UX fix)

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