Add option to keep controls expanded

[kohai-ut]

Closes #1604.

What this solves

The controller normally reveals the rewind, speed, and advance buttons on hover, then collapses again after use or a timeout. This adds an option for users who want those controls to remain continuously available.

Behavior

• Adds an unchecked Keep controls expanded preference: “Always show the rewind, speed, and advance buttons instead of revealing them only on hover.”
• Keeps the controls expanded after interaction and idle time when enabled.
• Applies preference changes immediately to controllers already open on the page; no reload is required.
• Leaves the existing behavior unchanged by default and for profiles where the setting is absent.
• Does not change the controller visibility toggle: the full controller can still be shown or hidden normally.
• Saves and syncs the preference with the other settings.
• Includes the preference in settings exports and restores it on import.

Validation

• Unit suite: 536 tests passed.
• Chrome display E2E: live preference changes, persistence after use/timeout, and restoration of hover behavior.
• Existing display visibility matrix: 96/96 states passed.
• Lint, formatting, build, and both TLC model checks passed.