Adds data migration support to the Settings page: a dedicated migrate button in Data Management, and a validation + migration prompt when importing files with missing or malformed fields.

Storage layer (storage.ts)

• ValidationResult interface — { isValid, hasPartialData, issues[], parsed }
• validateImportData(jsonString) — validates without importing; distinguishes fully valid / partially valid (missing required fields) / completely invalid (bad JSON, unrecognized object)
• buildMigratedData(data) (private) — merges partial AppData with defaults; applies existing category migration (per-type object → flat array); stamps latest version, schemaVersion, lastUpdated
• migrateAndImport(data) — runs partial data through migration and persists
• migrateToLatest() — migrates currently stored data in-place

UI layer (app.ts / index.html)

• Updated importData() — validates first; on partial data shows a confirm dialog listing issues before offering migration:

  Warning: The imported file has invalid or missing data:
    - Missing version field
  Would you like to migrate the valid data to the latest format?
  

• New migrateToLatest() — calls storage migration and reloads
• #migrateBtn added to Data Management section in Settings; safe to run at any time against current stored data

Tests

21 new unit tests covering validateImportData (valid, partial, bad JSON, non-array fields), migrateAndImport (defaults, old category format, existing arrays), and migrateToLatest (data preserved, version updated, missing fields filled).

• Fixes Data Update #46

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