High contrast, reduced motion, and text scaling preferences

[jonocodes]

Part of #6

What to build

The client respects user OS-level accessibility preferences. The theme automatically increases contrast when prefers-contrast: more is set, removes non-essential animation when prefers-reduced-motion: reduce is set, and scales typography with both the browser zoom and a user-controlled text-size setting already exposed in settings. Status is conveyed by more than color (icon + text + color), so users with color-vision differences can still distinguish states.

Acceptance criteria

• When prefers-contrast: more is set, chrome and widgets use a higher-contrast palette and thicker focus ring.
• When prefers-reduced-motion: reduce is set, transitions and scroll momentum animations are removed or shortened to a minimal duration.
• Status (connection, locked, transport state) is conveyed by an icon plus text in addition to color.
• Typography scales with browser zoom and the existing Text size setting without clipping or overlap.
• Manual check with macOS Increase Contrast, iOS Smart Invert, and prefers-reduced-motion in DevTools.

[jonocodes]

Implemented in 9049ecb and 14e779e. All AC met: under prefers-contrast: more the chrome and widgets use a higher-contrast palette with thicker borders and a wider black/white focus ring; under prefers-reduced-motion: reduce the connection-state pulse, media-icon playback dot, and press feedback's scale-down all stop animating; playback state is conveyed by icon + screen-reader text + colour (not colour alone); the four connection states are each colour-unique alongside their text labels; typography continues to scale with browser zoom + the existing Text-size setting. Coverage in a11y-css.test.ts, App.test.tsx, Tooltip.test.tsx.