feat(tariff): complete the progressive ladder — require open-ended last block, hours-based composer

The stepped-block engine already does "first N hrs x X, next N hrs x Y, ...,
24h cap" (ordered blocks, per-block rate, rolling-24h cap). No new axis; this
completes the model and removes its footgun.

- validateTariffStructure (shared) now REQUIRES the last block to be open-ended
  (uptoMin: null). A bounded final block silently inherited its own rate past
  its bound (a hidden, never-stated price — e.g. the live ALL tariff billed
  hour 4+ at the 3rd-hour rate). rateAt() still prices legacy bounded-tail
  versions; validation is publish-only, so published immutable versions are
  unaffected (no migration).
- TariffComposer edits bands as a DURATION in hours ("first 2 hours, then next
  3 hours"), accumulated into the engine's cumulative uptoMin (minutes) on
  submit. The last row is a pinned, non-removable "thereafter (open-ended)"
  band, so a published card always satisfies the open-ended-last rule.
  blocksToForm round-trips stored minutes back to band hours (legacy loads).
- i18n: replaced upToMin/egExample with bandDuration/hoursUnit/egHours (sq+en,
  catalog parity green).

Verified: validator rejects bounded-last / accepts open-ended; computeFee
correct at 1/2/3/5/6/24h for a 0-2h@200,2-5h@100,5h+@50 + 1000 cap card. Full
build green. Wiki (tariff.md, log.md) updated.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

apps/web/src/lib/i18n/en.ts

@@ -109,11 +109,12 @@ export const en: Catalog = {
     lostTicketFee: "Lost-ticket fee",
     exitGrace: "Exit walk-back grace (min)",
     rateBlocks: "Rate blocks",
-    rateBlocksHint: "Consumed in order as time accrues. \"Up to (min)\" is the block's upper bound; leave the last block's bound blank for \"thereafter\". Price is per billing increment.",
-    upToMin: "Up to (min)",
+    rateBlocksHint: "Each band lasts a number of hours and bills at its own price; bands are consumed in order (the first hours, then the next hours). The last band is \"thereafter\" (open-ended) — its price applies once the ladder is exhausted. Price is per billing increment.",
+    bandDuration: "Band duration",
+    hoursUnit: "hours",
+    egHours: "e.g. 2",
     pricePerIncrement: "Price / increment",
-    thereafter: "thereafter",
-    egExample: "e.g. 60",
+    thereafter: "thereafter (open-ended)",
     remove: "Remove",
     addBlock: "+ Add block",
     publishNewVersion: "Publish new version",