feat(cluster): configurable spot lifetime

Spots were bounded by COUNT alone, so on a quiet band a two-hour-old spot sat
on the band map looking like something to chase. Settings → DX Cluster now
takes a lifetime: presets at 5/10/15/30/60 minutes, or any value up to twelve
hours, and 0 keeps the old behaviour.

Spots are REMOVED from the shared list rather than hidden at render. The
cluster list, every band map and the counts all read that one array, so pruning
it once is what makes the setting mean the same thing in every view — the
lesson already paid for when the band map ignored the cluster's filters. It
also gives the memory back.

A spot whose timestamp will not parse is never dropped: an unreadable stamp is
a reason to distrust the clock, not to throw away the spot.

Swept every 30 seconds — close enough that a 5-minute setting is honoured, and
invisible next to the spot stream. Clamped in the backend as well as the UI.

frontend/src/lib/i18n.tsx

@@ -270,7 +270,7 @@ const en: Dict = {
   'clu.muteWorkedHint': '(they stay in the list, just quiet — leaves the colour for what is left to do)',
   'clu.slotHighlight': 'Colour the stations not worked on this band and mode',
   'clu.slotHighlightHint': '(by callsign, whatever the entity status says)',
-  'bo.open': 'open', 'bo.liveTip': '{band} is open — {n} stations, ~{km} km, {sector}{season}. Click for the band map.', 'bo.enable': 'Watch for band openings', 'bo.enableHint': '(10, 12, 6, 4 and 2 m. Switching this on adds the two RBN nodes and subscribes to the PSK Reporter feed — the detection needs far more ears than a cluster can give it.)', 'bo.feedUp': 'PSK Reporter feed up — {n} decodes seen', 'bo.feedDown': 'PSK Reporter feed down — needs your station grid, and a moment to connect', 'clu.chaseGrids': 'Chase new grids', 'clu.chaseGridsHint': '(learns locators from your own WSJT-X decodes AND from PSK Reporter, and keeps them in their own database so the cluster shows them from the first second)', 'clu.chaseGridsStat': '{n} locators known — {p} waiting to be written', 'clu.workedSameSlot': 'Already worked only on the same slot',
+  'bo.open': 'open', 'bo.liveTip': '{band} is open — {n} stations, ~{km} km, {sector}{season}. Click for the band map.', 'bo.enable': 'Watch for band openings', 'bo.enableHint': '(10, 12, 6, 4 and 2 m. Switching this on adds the two RBN nodes and subscribes to the PSK Reporter feed — the detection needs far more ears than a cluster can give it.)', 'bo.feedUp': 'PSK Reporter feed up — {n} decodes seen', 'bo.feedDown': 'PSK Reporter feed down — needs your station grid, and a moment to connect', 'clu.spotTtl': 'Spot lifetime', 'clu.spotTtlNever': 'Keep', 'clu.spotTtlHint': 'minutes — spots older than this are removed from the list and the band maps. 0 keeps them.', 'clu.chaseGrids': 'Chase new grids', 'clu.chaseGridsHint': '(learns locators from your own WSJT-X decodes AND from PSK Reporter, and keeps them in their own database so the cluster shows them from the first second)', 'clu.chaseGridsStat': '{n} locators known — {p} waiting to be written', 'clu.workedSameSlot': 'Already worked only on the same slot',
   'clu.workedSameSlotHint': '— a spot shows "worked" only if you worked that call on the SAME band and mode, not just anywhere. Combines with digital-mode grouping (Settings → General): with it on, a call worked on 20m FT8 also counts as worked for a 20m FT4 spot; with it off, FT8 and FT4 are separate slots.',
   // Backup panel
   'bk.hintMysql': 'On close (once/day) OpsLog snapshots the local SQLite (config) AND exports the shared MySQL log to ADIF — opslog-log-<date>.adi — so your contacts are protected even though they live on the server. Rotation keeps the last N of each.',