feat(economy): add /bank vault (rob-proof storage) + net-worth leaderboard

A strategic counterpart to /rob: coins moved to the bank are safe from /rob and
/heist (which only touch liquid balance) but earn no Bot Farm interest and can't
be spent, gambled or given until withdrawn - the deliberate trade-off against
keeping coins liquid.

- New bank.py (do_deposit/do_withdraw), bank_balance schema field.
- /bank (view), /deposit, /withdraw commands ('all' supported), with db_error
  handling; /balance shows the vault when non-zero.
- Coins leaderboard and /status money-supply now count net worth
  (balance + bank_balance), so banking never hides you from the board or the
  supply metric.
- Season reset wipes bank_balance too (no cross-season wealth hiding).

10 tests: deposit/withdraw math, guards, coin conservation, rob cannot touch the
vault, and net-worth accounting on the leaderboard + stats.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013VbAVsrZuYesea99mPMmPT

strings/__init__.py

@@ -60,6 +60,7 @@ from .economy import (
     CONSUMABLE_DESCRIPTIONS,
     VANITY_UI,
     LOOTBOX_UI,
+    BANK_UI,
     JAILED_UI,
     SHOP_BTN,
     DAILY_UI,
@@ -154,6 +155,7 @@ __all__ = [
     'CONSUMABLE_DESCRIPTIONS',
     'VANITY_UI',
     'LOOTBOX_UI',
+    'BANK_UI',
     'JAILED_UI',
     'SHOP_BTN',
     'DAILY_UI',