Enable Bengali localization and restore en.arb keys lost to a bad merge

[arnavgoel17]

Summary

Bengali translation work was sitting completely unused: app_bn.arb existed, but the generated Dart file was stale and — more importantly — bn was excluded from every user-facing surface because enabling it would have crashed. This PR makes Bengali a real, selectable language.

While validating with flutter gen-l10n, a second, larger problem surfaced:

98 keys had been silently dropped from app_en.arb by the conflict resolution in merge 79b5803 ("Merge branch 'main' into feat/ayurvedic-educational-layer"). Everything from welcomeToRhythma and all 17 phone+OTP login keys to the entire sms*, insightsNotEnoughData, deleteAccount and assistantAccessibility* families vanished from the source-of-truth ARB file while the generated Dart files kept them (hand-patched by bf2fbff to keep the app compiling). That hand-patching masked the loss — until anyone re-ran gen-l10n, which is exactly what this issue asks for.

What changed

• Restored the 98 lost keys to app_en.arb verbatim from pre-merge commit d1346cb.
• Completed Bengali coverage: added the 10 missing Ayurveda keys and translated the 5 strings still in English (insightsDisclaimer, logFlowVeryHeavy, logFlowSpotting, logSympSeverePain, logSympFainting). app_bn.arb now covers every key in app_en.arb.
• Regenerated all localization Dart files with flutter gen-l10n.
• Added bn (বাংলা) to appSupportedLanguages — both the onboarding language screen and Settings → Language iterate this list, so one entry enables it in both places, and LocaleProvider.setLocale now accepts it.
• Fixed three tests that encoded pre-change behavior (they could never run before, since the suite was blocked by compile errors):
  • supported_languages_test: expects 9 locales incl. bn
  • settings_screens_test: tapped the stale l10n-based label 'हिन्दी (Hindi)' instead of the current nativeName 'हिन्दी'
  • locale_provider_test: used a storage init API that no longer exists; migrated to the shared Hive test fixture, and added a new case proving setLocale(bn) is accepted

Validation

• flutter gen-l10n: clean regeneration; app_bn.arb has full key coverage of app_en.arb (verified programmatically)
• flutter analyze: 0 issues (baseline on main was 17 errors in committed generated files)
• flutter test: 118 passing, 0 failing
• Note: other locales still have untranslated keys relative to en (e.g. hi:38, mr:38, ta:28); those fall back to English at runtime and were out of scope here.

Fixes #297

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[ishita2740]

@arnavgoel17 I will merge the PR soon