feat(uploads): direct-to-B2 presigned image upload from iOS + Android

iOS (Swift) — primary path, since iOS is the live platform:
  - ImageDownsampler.swift: ImageIO/CGImageSourceCreateThumbnailAtIndex
    based resize. Pays only the cost of the resized bitmap rather than
    decoding the full source — a 12 MP iPhone photo previously
    materialized ~50 MB regardless of JPEG size. Profiles: completion
    (2048 px / quality 0.85), document_image (2560 px / 0.90).
  - PresignedUploader.swift: three-step orchestration (POST /uploads/presign
    → multipart POST direct to B2 with the signed policy fields → return
    upload_id). Maps HTTP errors to user-facing copy. Concurrent uploads
    via TaskGroup.
  - CompleteTaskView.swift: replaces the multipart-with-images path with
    downsample → upload-to-B2 → create-completion-with-upload_ids[]. The
    no-image branch unchanged.

Android (Kotlin) — parity:
  - composeApp/.../media/ImageDownsampler.kt: BitmapFactory inSampleSize
    + proportional scale + JPEG compress. Same profiles as iOS.
  - composeApp/.../network/UploadApi.kt: Ktor-based presign + direct-to-B2
    POST. Preserves form-field order so the S3 policy signature validates.
  - APILayer.uploadImage(category, contentType, bytes, fileName) → upload_id.
    UI integration to follow.

Shared (Kotlin):
  - models/TaskCompletion.kt: added uploadIds: List<Int>? to
    TaskCompletionCreateRequest and a new PresignUploadRequest /
    PresignUploadResponse pair matching the Go API DTOs.
  - Existing call sites (WidgetActionProcessor, PushNotificationManager)
    explicitly pass uploadIds: nil for backwards compatibility — Swift's
    bridge to Kotlin doesn't honor Kotlin defaults for required-positional
    parameters.

The legacy multipart path remains functional alongside the new one for
soak-test purposes; per-platform feature flags can flip between them at
any time. After zero multipart traffic in production for 7 consecutive
days, the legacy paths can be dropped.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

iosApp/iosApp/PushNotifications/PushNotificationManager.swift

@@ -388,7 +388,8 @@ class PushNotificationManager: NSObject, ObservableObject {
                     notes: nil,
                     actualCost: nil,
                     rating: nil,
-                    imageUrls: nil
+                    imageUrls: nil,
+                    uploadIds: nil
                 )
                 let result = try await APILayer.shared.createTaskCompletion(request: request)