Fix RequestView over-funding reentrancy (do_give double-transfer)

A follow-up reentrancy sweep of the five remaining money-moving views found
one more instance of the same bug class. The other four (quests, prestige,
fish, shop) verified clean - their underlying do_* functions are idempotent.

RequestView / FundModal (commands/economy_support_commands.py):
- on_submit read self._view.remaining, then awaited do_give, then decremented
  remaining. Because discord.py dispatches each modal submit as its own task
  and do_give is a plain non-idempotent transfer, a funder could open two
  modals and submit both before the first resolved: both read the same
  pre-decrement remaining, both passed the range check, and both transferred
  `amount` - over-funding the request (remaining goes negative) and moving up
  to the funder's whole balance.
- Fix: reserve the amount synchronously (decrement remaining BEFORE the do_give
  await, with no await in between - atomic under asyncio), and roll the
  reservation back if the transfer fails. The second concurrent submit now
  sees the reduced remaining and is rejected. Added a cheap _fund guard
  (remaining<=0 / is_finished) so a click on a funded request doesn't open a
  dead modal.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

strings.py

@@ -674,6 +674,7 @@ ERR: dict[str, str] = {
     "request_self":      "❌ Sa ei saa iseendalt anuda.",
     "request_bot":       "❌ Botid on teatavasti kitsid.",
     "request_targeted":  "❌ See taotlus on suunatud kasutajale **{name}**.",
+    "request_closed":    "❌ See taotlus on juba rahastatud või aegunud.",
     "already_in_game":   "❌ Sul on juba aktiivne mäng käimas!",
     "invalid_amount":    "❌ Sisesta kehtiv summa või 'all'.",
     "fund_range":        "❌ Sisesta summa vahemikus 1-{max}.",