feat: Comprehensive accessibility improvements for screen readers

Major enhancements to screen reader support throughout the application:

Focus Management & Navigation
- Enhanced WizardStepBase to automatically make titles and subtitles focusable
- Registered header elements as first focus group with negative order (-100)
- Ensured consistent focus flow: Title, Subtitle, Content, Buttons
- Removed manual initialFocusItem overrides in favor of automatic header focus

Selection Steps
- Added dynamic item counts to list announcements (OS, Device, Storage)
- Fixed double-reading of delegate descriptions by marking visual text as ignored
- Made individual labels focusable in summary grids
- Added contextual status messages for storage selection (e.g., no devices, read-only)
- Made "no devices available" message invisible but accessible

Customization Steps
- Applied label pattern: moved descriptions from fields to focusable labels
- Made labels focusable and placed before their corresponding controls
- Enhanced descriptions to explain why selections are made (e.g., capital city)
- Added accessibility support for all form labels and help text
- Made WizardDescriptionText focusable by default

Dialogs & Warnings
- Made dialog titles, messages, and warning text focusable
- Added proper focus groups to system drive warning dialogs
- Ensured critical warnings are read before action buttons
- Made write confirmation dialog text fully accessible

Writing & Completion Steps
- Made progress text focusable for real-time status updates
- Dynamic subtitle during writing: "Writing in progress - do not disconnect the storage device"
- Made summary items individually focusable (device, OS, storage)
- Enhanced customizations list to announce all applied items
- Made eject instruction focusable on completion screen

WiFi Step
- Simplified confirmation field behavior to match user customization step
- Always include label and field in focus order (no conditional enabling)
- Removed confirmNeedsInput complexity in favor of simpler validation

Bug Fixes
- Fixed keyboard navigation for "Apply & Restart" button in Repository dialog
- Fixed keyboard navigation for "Open Raspberry Pi Connect" button
- Fixed confirmation password label being skipped in WiFi step
- Fixed focus order rebuilding when UI state changes

47 files changed, 10993 insertions(+), 1145 deletions(-)

Author: tdewey-rpi