Address the review follow-ups from #2188, #2189 and #2190 (#2193)

* Read the collapsed-threads key through getJsonItem

`getFromLocalStorage` parsed the stored string directly, so anything
malformed under `__remarkCollapsed` threw out of `restoreCollapsedThreads`.
That call sits in `remark.tsx` ahead of the `render`, so the throw took the
whole widget with it: the reader was left on the preloader, over a view
preference.

`getJsonItem` in `common/local-storage.ts` already wraps a parse of a
localStorage key and returns null on failure, and null is a shape the check
below already reads as empty. The rest of that function is total against
whatever the browser holds, and the bare parse was the one way in.

* Stop retrying a failed e2e test in CI

The suite went in with one gotestsum rerun. It has no failures on record to
justify that: 31 CI runs since it landed, all green, and no rerun report has
ever been produced. A retry is what turns an intermittent regression into a
green build, and while the suite is this young its own failures are the
evidence worth keeping.

`E2E_RUN_ID` stays. It stamps the threads a run works on with the CI run id, so
a thread url in a trace or a log names the run it came from. It carries no data
across: the stack is disposable, and a local run under the same id gets those
urls on an empty database.

* Stop two chooseUnusedPort comments claiming collisions cannot happen

All four copies listen on :0, read the assigned port, close the listener
and bind later, so nothing holds the number across that gap and another
binary can take it. The copies in app/cmd and app/rest/api call a collision
very unlikely, which is accurate; the ones in app and the example module
said binaries never land on the same number, which is not, and a comment
ruling out a port collision is what would send the next person chasing one
somewhere else. All four now read the same.

Closing the window rather than describing it means the server binding :0
itself and reporting the address it got, which is a larger change.

backend/_example/memory_store/server/rpc_test.go

@@ -19,8 +19,8 @@ import (
 	"github.com/umputun/remark42/memory_store/accessor"
 )
 
-// chooseUnusedPort asks the kernel for a free port from the ephemeral range, so concurrently
-// running package test binaries never land on the same number
+// chooseUnusedPort asks the kernel for a free port from the ephemeral range, which makes a
+// collision between concurrently running package test binaries very unlikely
 func chooseUnusedPort(t *testing.T) int {
 	t.Helper()
 	ln, err := net.Listen("tcp", ":0")