fix(uploads): switch from S3 multipart POST to presigned PUT

Backblaze B2's S3-compatible endpoint does not implement the S3 POST
Object operation — every POST returns HTTP 501 regardless of URL form
(path-style or virtual-hosted-style). The previous multipart-POST flow
has been failing for every task-completion image upload.

Server-side companion change (honeyDueAPI master @7cc5448) replaces
PresignedPostPolicy with PresignHeader/PUT and renames the response
field from "fields" to "headers". This commit aligns both clients.

PresignUploadResponse model: field renamed `fields` → `headers`,
added `method` (default "PUT"). Both new fields have defaults so a
build talking to a stale server still decodes — albeit with empty
headers, which would then 403 at signature time. The server is
already on the new shape in prod.

iOS PresignedUploader.swift: dropped the ~70-line multipart body
builder and S3 form-field ordering logic. Replaced with a single PUT
request that applies server-supplied headers verbatim (skipping
Content-Length, which URLSession sets automatically and refuses to
override).

Android UploadApi.kt: same shape change. `postToStorage` →
`putToStorage`. Single Ktor `client.put()` with headers passthrough.
`uploadOne`'s `fileName` parameter kept for source compatibility but
marked @Suppress("UNUSED_PARAMETER") since PUT doesn't need it.

Verified end-to-end against api.myhoneydue.com:
  presign → PUT 12 bytes → HTTP 200 in 0.6s.

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

composeApp/src/commonMain/kotlin/com/tt/honeyDue/models/TaskCompletion.kt

@@ -34,15 +34,20 @@ data class PresignUploadRequest(
 /**
  * Presigned upload session — response from POST /api/uploads/presign.
  *
- * The client uses [uploadUrl] + [fields] to perform a multipart/form-data
- * POST directly to B2, then passes [id] back in the upload_ids[] field of
- * the next /api/task-completions/ or /api/documents/ create call.
+ * The client makes one PUT request to [uploadUrl] with the raw object
+ * bytes as the body and [headers] as the request headers. On success,
+ * pass [id] back in the upload_ids[] field of the next
+ * /api/task-completions/ or /api/documents/ create call.
+ *
+ * PUT (not POST) because B2's S3-compatible endpoint does not implement
+ * the S3 POST Object form upload (returns HTTP 501).
  */
 @Serializable
 data class PresignUploadResponse(
     val id: Int,
     @SerialName("upload_url") val uploadUrl: String,
-    val fields: Map<String, String>,
+    val method: String = "PUT",
+    val headers: Map<String, String> = emptyMap(),
     val key: String,
     @SerialName("expires_at") val expiresAt: String
 )