profiler: check a profile against a stored card snapshot

Each profile row gets a Check card snapshot button next to Check card.
It opens a picker listing the stored snapshots (name, ICCID, date, file
count) and runs the profile rules offline against the selected snapshot,
showing the usual report titled 'profile — snapshot'.

profilerRunRule() accepts an optional data source: profilerLiveSource()
(default) wraps /api/select + /api/read, profilerSnapshotSource() serves
select/read from the snapshot files. Contents missing from the snapshot
are reported as unverifiable errors ('Content not captured in snapshot')
rather than mismatches. The shared check loop moved into
profilerRunProfile().

Tests for the snapshot source and snapshot-based checks; help docs
updated; SW cache v84 -> v85.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'otaman-v84';
+const CACHE = 'otaman-v85';
 const URLS = [
   'index.html',
   'help.html',