refactor(web): harden oauth state store and session writes after review

- Drop the FIFO from oauthStates: consumed states left entries behind, so
  an unrate-limited start/cancel cycle grew the slice without bound.
  Evict by oldest expiry instead — the map alone now bounds memory.
- CreateSession runs INSERT + expiry sweep in one transaction.
- slices.Contains replaces a hand-rolled contains; APIBase typo fixed.
- Stale comments and test paths updated; login hover uses --ember-ink.

backend/internal/session/session.go

@@ -95,7 +95,7 @@ func ClientIP(r *http.Request) string {
 // Behind carrier-grade NAT this budget is shared with every other subscriber on
 // the same public address, so a stranger can lock the owner out for up to one
 // window. That is accepted: the block self-heals, and ten attempts is generous
-// for a mistyped password.
+// for the occasional fumbled sign-in.
 //
 // State is in memory and per-process, so a restart clears it. Entries are
 // pruned lazily on access; for a single-user deployment the map cannot grow