Implement Phase 2: end-to-end import system

Complete the import flow from file upload through duplicate resolution
to transaction creation. Previously, ResolveDuplicatesAsync was a stub
that only incremented counters, FileWatcherService detected files but
didn't process them, and ImportsView.vue ignored the API response.

Backend:
- Add ParsedDataJson column to ImportBatch to persist parsed data
  between upload and confirm steps
- Rewrite ImportService.ResolveDuplicatesAsync to deserialize parsed
  transactions and create/merge/skip based on user decisions, with
  payee matching, fingerprinting, and account balance updates
- Add GetBatchesAsync for import history
- Complete FileWatcherService: wait for file ready, resolve account by
  filename, call UploadAsync, send SignalR notification, archive file
- Add GET /api/imports/batches endpoint

Frontend:
- Create imports API service (frontend/src/services/imports.ts)
- Add ImportBatch, DuplicateResolution types
- Rewrite ImportsView.vue with 3-step Vuetify stepper wizard:
  upload, review with duplicate handling, completion summary,
  plus import history table

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Purrse.Core/Models/ImportBatch.cs

@@ -14,6 +14,7 @@ public class ImportBatch
     public int DuplicateCount { get; set; }
     public int SkippedCount { get; set; }
     public ImportStatus Status { get; set; } = ImportStatus.Pending;
+    public string? ParsedDataJson { get; set; }
     public DateTime ImportedAt { get; set; } = DateTime.UtcNow;
 
     public User User { get; set; } = null!;