[Desktop] Improve accessibility labels and focus order

[TheZupZup]

Part of #376.

Goal

Audit desktop controls for meaningful semantics, predictable focus order and accessible actions.

Likely code areas

• shared widgets/semantics wrappers
• lib/features/library/
• lib/features/player/
• desktop navigation/dialog/settings widgets
• accessibility widget tests

Requirements

• Prioritize playback controls, sidebar navigation, track rows, dialogs and server-management surfaces.
• Provide labels/state/value semantics for icon-only controls and sliders.
• Ensure focus order follows the visual/logical layout and remains usable in multi-pane views.
• Do not duplicate visible text unnecessarily for screen readers.
• Keep semantics useful on Android too when the widget is shared.

Acceptance criteria

• Major interactive desktop controls expose useful names/roles/states.
• Focus order is predictable with keyboard navigation.
• Disabled/selected/playing states are communicated appropriately.
• Icon-only playback controls are understandable without vision.
• Representative semantics/focus tests are added.