fix(frontend): the scheduler's two number editors stop swallowing what you typed

Both refused an out-of-range value by discarding it: the entry vanished, the box
snapped back to the server's number, and nothing said whether the save had
happened, been rejected, or been ignored. That is the house rule about honest
states, applied to a write instead of a read. They now keep the draft, mark the
field aria-invalid and name the range (HU+EN), and the Save control is disabled
rather than silently inert.

The interval editor also dropped a keyboard user's focus on every exit — Escape,
Cancel and Save all unmount the focused input — so it hands focus back to the
pencil that opened it, the pattern IssueComposer already uses.

Tests: the four interval-editor cases added in S2 are relabelled CHARACTERIZATION,
because they pass against the pre-sweep code too (the old effect was already
guarded by `editing`). The editor that genuinely lost typing to a poll is the
maintenance batch box, whose old effect had no guard at all, and it had no such
test — it does now, anchored on a value that follows the server so it cannot pass
on a page that simply has not polled yet.

frontend/src/i18n/locales/en/scheduler.json

@@ -23,6 +23,7 @@
   "quotaRemaining": "{{formatted}} left",
   "quotaNote": "Shared YouTube Data API budget; resets at midnight US Pacific. Backfill yields to leave headroom for interactive use.",
   "minutes": "min",
+  "range": "Must be between {{min}} and {{max}}.",
   "editInterval": "Click to change how often this runs",
   "intervalFailed": "Couldn't change the interval",
   "runNow": "Run now",

frontend/src/i18n/locales/hu/scheduler.json

@@ -23,6 +23,7 @@
   "quotaRemaining": "{{formatted}} maradt",
   "quotaNote": "Közös YouTube Data API-keret; éjfélkor (US Pacific) nullázódik. A backfill hátrébb sorol, hogy maradjon hely az interaktív használatnak.",
   "minutes": "perc",
+  "range": "{{min}} és {{max}} között lehet.",
   "editInterval": "Kattints a gyakoriság módosításához",
   "intervalFailed": "Nem sikerült módosítani a gyakoriságot",
   "runNow": "Futtatás most",