feat(app): add global text size setting and meal plan calendar export

[rjmlaird]

Summary

This PR adds a global text size preference for the app and a weekly meal plan export to calendar. It also wires the new settings into app startup, updates the theme to respect the selected scale, and adds the Android file-sharing plumbing needed for .ics export.

What changed

Text size

• Added a new TextScale setting stored locally in DataStore.
• Exposed the setting through SettingsRepository, ThemeViewModel, and SettingsViewModel.
• Updated OpenCookTheme to apply the chosen text scale on top of the system font scale.
• Added a new settings row and dialog so users can choose Normal, Large, or Extra large text.
• Added localized strings and unit tests for text-scale parsing and behavior.

Meal plan export

• Added .ics export support for the visible weekly meal plan.
• Created IcsExport to generate RFC 5545 calendar output with one all-day event per planned day.
• Added IcsShare plus a FileProvider so the export can be shared safely through Android.
• Added a calendar export action to the meal plan screen.
• Added localized strings and unit tests for export formatting, escaping, and UID stability.

Notes

• The text size setting is device-local and not synced.
• The calendar export is a one-shot file export, not a live subscription feed.

Testing

• Verified TextScale persistence and fallback behavior with unit tests.
• Verified .ics generation with tests for empty weeks, date ranges, escaping, and CRLF line endings.
• Manually checked that the settings screen exposes the new text size option and that meal plan export triggers a share intent.

Files touched

• MainActivity.kt
• SettingsRepository.kt
• ThemeViewModel.kt
• SettingsViewModel.kt
• SettingsScreen.kt
• Theme.kt
• MealPlanScreen.kt
• IcsExport.kt
• IcsShare.kt
• AndroidManifest.xml
• file_paths.xml
• strings.xml
• strings-de.xml
• TextScaleTest.kt
• IcsExportTest.kt