Merge RB-25 — add the UPLOAD_TRANSPORT injection token

TE-003: UploadShellService documented UploadTransport as the swap seam, then
bound the concrete, unexported KeepaliveTransport class directly, so a spec
could not fake it. UPLOAD_TRANSPORT copies the SESSION_PORT shape; the
default factory returns the same instance, so runtime behaviour is
unchanged. upload-shell.service.ts goes from 0% to 88.57% line coverage
across 16 new specs for upload(), cancel(), delete() and pollReturning().

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

# Conflicts:
#	docs/project/refactor-backlog-setup/refactor-backlog/99-backlog.md
#	libs/shared/docs/behaviour-spec.mdx

libs/shared/docs/behaviour-spec.mdx

@@ -20,7 +20,7 @@ tested where._
 
 Every bullet below is a real test name from the suite — an `it()` title (frontend) or a test
 method name (backend), read as a sentence. Nothing here is hand-written prose: this page
-**is** the suite, reshaped for a business reader. 474 frontend behaviours across
+**is** the suite, reshaped for a business reader. 487 frontend behaviours across
 9 contexts; 261 backend behaviours across 42 test
 classes.
 
@@ -675,6 +675,31 @@ classes.
 - map only touches Success
 - map2 precedence: Failure > Loading > Success
 
+#### UploadShellService.cancel
+
+- calls the transport cancel function for an in-flight upload and forgets it
+- is a no-op for a localId with nothing in flight
+
+#### UploadShellService.delete
+
+- dispatches UploadDeleting, then UploadDeleteComplete on success
+- dispatches UploadDeleteFailed with the server detail on failure
+- falls back to an empty reason when the server sends no detail
+
+#### UploadShellService.pollReturning
+
+- does nothing when there are no uploads to poll
+- dispatches BackgroundUploadsReturned for uploads the server reports complete
+- does not dispatch when nothing has arrived yet
+
+#### UploadShellService.upload
+
+- dispatches UploadQueued with the transport backgroundSync flag, then sends via the transport
+- translates a progress callback into UploadProgress
+- translates a resolved transport into UploadComplete
+- translates a rejected transport into UploadFailed with the reason
+- does not dispatch UploadFailed on a user-initiated abort
+
 #### authGuard
 
 - allows an authenticated user