feat: accessibility improvements and settings reorganization

[jesposito]

Summary

Comprehensive accessibility audit and settings panel improvements:

• WCAG 2.1 AA compliance - Modal focus trapping, keyboard navigation, screen reader support
• Missing features from upstream - Schedule settings and Quality (CRF) sliders
• Settings reorganization - Collapsible sections for cleaner UI

Accessibility Changes

Modals

• Add role="dialog", aria-modal="true", aria-labelledby to all modals
• Implement focus trap with inert attribute on background content
• Restore focus to previously focused element on close
• Unified Escape key handling for all modals

Keyboard Navigation

• Skip link for bypassing header content
• Tab panels support Arrow/Home/End key navigation
• Preset cards now role="button" with Enter/Space/Arrow key support
• Collapsible sections keyboard accessible

Screen Reader Support

• Loading state announcements via aria-live region
• Proper aria-label on all form inputs and icon buttons
• Decorative SVGs marked aria-hidden="true"

Visual

• Improved color contrast for tertiary text (WCAG AA)
• prefers-reduced-motion CSS media query support
• 44px minimum touch targets for checkboxes

Settings Panel Changes

New Features (from gwlsn upstream)

• Schedule - Enable/disable scheduling with time window picker
• Quality - HEVC and AV1 CRF sliders with numeric inputs
• Schedule status - Shows active schedule in header

Reorganization

• Consolidated into logical groups: Transcoding, Schedule, Quality, Notifications, Interface
• Quality and Notifications sections are collapsible (collapsed by default)
• Removed duplicate "Transcoding" section headers

Tests

Expanded e2e/accessibility.spec.ts from 3 tests to ~30 tests covering:

• Document structure and skip link
• Modal ARIA attributes and keyboard interaction
• Tab navigation and panel roles
• Form control labeling
• Collapsible section accessibility