refactor: add successOr, sweep remaining inline unwraps (RD-17)

Eight sites hand-rolled `rd.tag === 'Success' ? rd.value : fallback`. Six
take the new `successOr(rd, fallback)`, one takes the existing `successOf`,
and one (`big-profile.store.ts`) uses the existing `map`, since it returns a
RemoteData rather than an unwrapped value.

Co-Authored-By: Claude Sonnet 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. 530 frontend behaviours across
+**is** the suite, reshaped for a business reader. 532 frontend behaviours across
 9 contexts; 261 backend behaviours across 42 test
 classes.
 
@@ -968,6 +968,11 @@ classes.
 - unwraps a Success value
 - is undefined for every other state
 
+#### successOr
+
+- unwraps a Success value
+- is the fallback for every other state
+
 #### upload lifecycle messages
 
 - queued → progress → complete