feat: widget size variants, vault selection, per-instance pinning, concurrent cache tests (#244 #245 #246 #247)

[edwardfavour998-code]

• Add Small/Medium/Large Widget Size Variants With Adaptive Layout #247 iOS: TTLWidgetView adapts layout across .systemSmall/.systemMedium/.systemLarge/.accessoryRectangular/.accessoryCircular
• Add Small/Medium/Large Widget Size Variants With Adaptive Layout #247 Android: vault_widget_small/medium/large.xml layouts; VaultStatusWidget picks layout by width; onAppWidgetOptionsChanged re-renders on resize
• Implement Widget Vault-Selection UI Instead of Always Showing the First Vault #245 iOS: VaultSelectionIntent (AppIntents/WidgetConfigurationIntent) replaces StaticConfiguration; urgency fallback when vaultID is empty
• Implement Widget Vault-Selection UI Instead of Always Showing the First Vault #245 Android: VaultWidgetConfigActivity lets user pick a vault per widget instance; vault_id_list saved by worker for picker; urgency fallback on cancel
• Add Widget Configuration to Pin a Specific Vault #246 iOS: AppIntentConfiguration gives each widget instance its own VaultSelectionIntent automatically
• Add Widget Configuration to Pin a Specific Vault #246 Android: per-widget SharedPreferences (vault_widget_prefs_{id}); VaultWidgetUpdateWorker updates each widget independently
• Add Tests for OfflineCache Behavior Under Concurrent Read/Write #244 Android: OfflineCacheTest concurrent read/write test; OfflineCache save/load protected by ReentrantReadWriteLock
• Add Tests for OfflineCache Behavior Under Concurrent Read/Write #244 iOS: OfflineCacheConcurrencyTests (concurrent write+read, two-instance isolation); OfflineCache.save/load/delete/clearAll serialised via concurrent DispatchQueue with barrier writes
• Updated PARITY.md: widget urgency selection now ✅ on both platforms
• Updated .gitignore: exclude snapshot images, build artefacts, generated Xcode project

Closes #244
Closes #245
Closes #246
Closes #247

Summary

Changes

Testing

Parity checklist

This project maintains a feature-parity table in PARITY.md that
tracks which features are implemented on iOS vs Android. Please answer the
questions below before requesting review.

• This PR does not add, change, or remove any user-facing feature on either
  platform — no PARITY.md update needed.

  — OR —

• This PR adds/changes/removes a user-facing feature. I have updated PARITY.md:

  • Updated the status symbol(s) for the affected row(s).
  • Added or updated "Notes" if the implementation is partial or has caveats.
  • Removed or updated any rows in the "Known gaps" table that this PR closes.

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