feat: add cache-purge pipeline on account switch

[Timrossid]

Scope

Fixes stale contract state and cached RPC responses persisting when users switch between multiple Stellar accounts.

Changes

WalletProvider (\src/components/providers/WalletProvider.tsx)

• cacheVersion ref exposed via \WalletContext\ — descendant hooks can observe it before resolving stale promises
• purgeCache() pipeline atomically clears React context state (\�ccount, \�ccounts), localStorage entries (\utility-wallet-session, \utility-wallet-accounts, \utility-auth-session), IndexedDB stores scoped to the old account key prefix, and aborts in-flight RPC requests
• switchAccount guard serializes execution via a module-level promise chain (\switchGuard) to prevent concurrent switch race conditions
• disconnect updated to reuse the same \purgeCache\ pipeline

api.ts (\src/services/api.ts)

• Added \�ctiveControllers\ Set tracking all in-flight \AbortController\ instances
• Added \�bortAllRequests()\ exported function to abort all active requests
• 
  equest()\ registers/unregisters its controller via \inally\ block for reliable cleanup

cache.ts (\src/services/cache.ts)

• Added \cacheClearByPrefix(prefix)\ to delete IndexedDB entries matching a given key prefix

Resolution Strategy

1. \cacheVersion\ (useRef) incremented on every purge and exposed via context
2. \switchAccount\ wraps inner logic with \switchGuard\ promise chain (mutex pattern)
3. \purgeCache\ is called inside the mutex, passing the old account address for scoped IndexedDB cleanup
4. \�bortAllRequests()\ aborts all tracked AbortControllers from \�pi.ts\ on each switch

Closes #1