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be spent, gambled or given until withdrawn - the deliberate trade-off against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/economy.py

@@ -19,6 +19,7 @@ __all__ = [
     'CONSUMABLE_DESCRIPTIONS',
     'VANITY_UI',
     'LOOTBOX_UI',
+    'BANK_UI',
     'JAILED_UI',
     'SHOP_BTN',
     'DAILY_UI',
@@ -275,6 +276,20 @@ LOOTBOX_UI: dict[str, str] = {
     "foot_buff":     "Saldo: {balance}",
 }
 
+# ---------------------------------------------------------------------------
+# Bank vault  (rob-proof storage; liquid vs banked coins)
+# ---------------------------------------------------------------------------
+BANK_UI: dict[str, str] = {
+    "title":        "🏦 TipiPANK",
+    "desc":         "Panka pandud mündid on **röövikindlad** (`/rob` ja `/heist` neid ei puuduta), aga neid ei saa kulutada ega panustada enne väljavõtmist ega teeni Botikoopa intressi.",
+    "f_liquid":     "💵 Rahakotis (vaba)",
+    "f_bank":       "🏦 Pangas (kaitstud)",
+    "deposited":    "🏦 Panid **{amount}** panka.\n💵 Rahakotis: {balance}  ·  🏦 Pangas: {bank}",
+    "withdrawn":    "💵 Võtsid **{amount}** pangast välja.\n💵 Rahakotis: {balance}  ·  🏦 Pangas: {bank}",
+    "nothing_liquid": "❌ Sul pole nii palju vaba raha rahakotis.",
+    "nothing_bank":   "❌ Sul pole nii palju raha pangas.",
+}
+
 JAILED_UI: dict[str, str] = {
     "title":      "🔒 Praegu vanglas",
     "empty":      "Kõik on vabad! Vanglas pole kedagi.",