Fix backup state logic, ring colors, Already Safe invariant, and dedup

BackupJob: fix finish() — failedCount > 0 now correctly sets .failed
instead of .completed in both branches.

BackupEngine: replace filename-only fileExists check with a 3-layer
dedup strategy. For each asset: (1) check ManifestIndex by localIdentifier
O(1); (2) filename fallback for bootstrap manifests; (3) NAS fileExists
for files not yet in manifest. Never overwrite an existing NAS file.
Manifest index is built once after connect and held in a local let for
the loop — Data+BackupManifest are released immediately after.

BackupView — ring color:
  active (running/preparing/paused) → orange always
  failed → destructive
  needBackup == 0 && phoneTotal > 0 → green
  default → neutral gray
  Ring is no longer permanently green after .completed.

BackupView — CTA button:
  Default color is always black (AppTheme.ink).
  On .completed: briefly flashes green for 2.5 s then smoothly
  reverts to black. ctaFlashGreen state drives the color.

BackupView — Already Safe invariant:
  alreadySafeDisplayCount = min(base + uploadedFiles, phoneTotal)
  during active backup; min(base, phoneTotal) otherwise.
  Adding skippedFiles was removed — they are already counted in
  snapshot.alreadySafe from the last reconcile.
  notBackedUpDisplayCount is derived from alreadySafeDisplayCount to
  keep the invariant alreadySafe + needBackup == phoneTotal.

Co-Authored-By: Kutesir <kutesir@provoc.ug>
Co-Authored-By: Sentry <sentry@provoc.ug>

Core/Models/BackupJob.swift

@@ -72,7 +72,9 @@ final class BackupJob: ObservableObject {
 
     func finish(result: BackupResult) {
         endDate = Date()
-        status = result.failedCount == 0 ? .completed : .completed
+        status = result.failedCount == 0
+            ? .completed
+            : .failed("Completed with \(result.failedCount) error\(result.failedCount == 1 ? "" : "s")")
     }
 
     func fail(error: BackupError) {