feat(android): add JSON share intent for AI-assisted meal import

[Marc4k]

Summary

Adds Android share intent support so any app — including AI assistants
using the bundled food-tracker skill — can share meal JSON directly to
OpenNutriTracker. The skill accepts natural-language descriptions and
food photos; it generates the app's existing JSON export format and the
user taps Share → OpenNutriTracker, confirms the import dialog, and the
meals land in their diary.

iOS share intent is out of scope for this PR (no Share Extension target
exists yet). The Flutter service returns null on iOS so there is no
regression — tracked as a follow-up.

Ai Skill

food-tracker.skill.zip

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Documentation
• CI / tooling
• Localization
• Other

Related issues

Changes

• Android — AndroidManifest.xml registers ACTION_SEND intent-filters
  for text/plain and application/json so the app appears in the system
  share sheet
• Android — MainActivity.kt captures the shared text via a
  MethodChannel and holds it until Flutter reads it (handles both cold-start
  and singleTop onNewIntent)
• Flutter — ShareIntentService bridges the channel; returns null on
  iOS (no-op fallback)
• Flutter — MainScreen checks for a pending share on cold start and on
  every foreground resume via AppLifecycleListener, shows a confirmation
  dialog, and imports approved meals using the existing
  ImportMealsJsonUsecase
• l10n — two new strings (shareJsonImportContent,
  shareJsonImportErrorLabel) added to all 9 supported locales (EN + DE, CS,
  IT, PL, SK, TR, UK, ZH)
• fix — pre-existing curly_braces_in_flow_control_structures lint in
  meal_relevance_ranker.dart that blocked CI

Screenshots / recordings

example.mp4

Test plan

• Described steps below were followed locally
• Unit / widget tests added or updated (if applicable)
• Manual check on Android
• Manual check on iOS (if applicable) — N/A, iOS not yet supported

Steps

1. Build and install the app (fvm flutter run --flavor develop)
2. Use the food-tracker AI skill to generate a meal — it outputs JSON in
   ONT's export format
3. In the AI app, tap Share → OpenNutriTracker
4. Confirm the import dialog — meals appear in today's diary
5. Share invalid/non-JSON text to ONT — error snackbar is shown, no crash

Checklist

• Code follows project style (just format / 120-char line width)
• New interactive widgets include Semantics(identifier: '...') where
  needed (share-json-import-confirm on the dialog confirm button)
• Localization updated in ARBs and lib/generated/ when user-facing
  strings change
• Codegen ran if DBOs/DTOs/env changed (just build) — no codegen changes
• No secrets or .env values committed
• PR title follows conventional commit style (e.g. feat:, fix:, chore:)

[Marc4k]

@simonoppowa, any updates on this one? I use it daily and would love to switch back to the main app instead of running my fork. Happy to also implement iOS support if that's a blocker for merging.