feat: improve application accessibility

[zefutoff]

Summary

• add semantic accessibility headings to the main application screens
• announce dynamic correction and status changes through polite accessibility live regions
• enforce minimum 48 dp touch targets for IME controls and suggestions
• adapt the IME layout automatically for large font scales
• stack header and correction actions when fontScale >= 1.5
• keep suggestion chips horizontally scrollable
• add instrumentation coverage for accessibility semantics
• complete the Phase 4 accessibility roadmap item

Large-text support

The correction IME now switches to a stacked layout for large accessibility text:

• Grammalecte remains readable without wrapping
• the return-keyboard action moves below the title
• automatic correction actions use the full available width
• long actions such as Corriger tout et revenir are no longer truncated

Validation

• TalkBack manually validated on Samsung Galaxy S21
• navigation order manually validated
• accessibility headings manually validated
• dynamic status announcements manually validated
• suggestion buttons manually validated with TalkBack
• main application UI manually validated at font_scale=2.0
• correction IME manually validated at font_scale=2.0
• AccessibilityInstrumentedTest passes
• make check
• make assemble
• git diff --check
• final APK does not request android.permission.INTERNET

Roadmap

Completes:

• Improve accessibility of the application UI