Implement Phase 6: automated bank sync (Plaid + SimpleFIN) & settings

Add automated transaction syncing via Plaid and SimpleFIN providers,
a Settings page for managing connections and account mappings,
background scheduled syncing, and sync history logging.

New backend: BankSync plugin project with PlaidSyncProvider and
SimpleFinSyncProvider, SyncConnection/LinkedAccount/SyncLog models,
AES-256 encryption service, BankSyncService orchestration,
BankSyncBackgroundService (15-min tick), and BankSyncController
(13 endpoints). EF migration creates sync_connections,
linked_accounts, and sync_logs tables.

New frontend: Settings view with 3 tabs (Profile, Bank Connections,
Sync History), bankSync API service, BankSyncComplete SignalR
notifications, and Settings nav item in sidebar + avatar dropdown.

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

src/Purrse.Api/appsettings.json

@@ -16,6 +16,16 @@
   "Imports": {
     "WatchPath": "imports"
   },
+  "BankSync": {
+    "EncryptionKey": "CHANGE_ME_32_CHAR_KEY_FOR_AES256!",
+    "Plaid": {
+      "ClientId": "",
+      "Secret": "",
+      "Environment": "sandbox"
+    },
+    "DefaultSyncIntervalMinutes": 360,
+    "BackgroundCheckIntervalMinutes": 15
+  },
   "Logging": {
     "LogLevel": {
       "Default": "Information",