refactor(shared): add UPLOAD_TRANSPORT injection token (RB-25)

UploadShellService injected the concrete KeepaliveTransport class instead
of a token. The class was not exported, so a spec could not fake it, and
could not provide against the UploadTransport interface either, since an
interface is not a DI token. The port existed only on paper.

Add UPLOAD_TRANSPORT, an InjectionToken with a default factory that
resolves the same KeepaliveTransport singleton, following the
SessionPort/SESSION_PORT shape. UploadShellService now injects the token.
Runtime behaviour is unchanged.

Add upload-shell.service.spec.ts: a recording fake transport plus a fake
UploadAdapter exercise upload(), delete(), cancel() and pollReturning(),
the four methods the missing seam left unreachable. Coverage for
upload-shell.service.ts goes from 0% to 88.6% line / 85% branch.

Mark RB-25 done in 99-backlog.md and add its implementation note.

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

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. 467 frontend behaviours across
+**is** the suite, reshaped for a business reader. 480 frontend behaviours across
 9 contexts; 238 backend behaviours across 41 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