[SDK · Account management] Implement AccountManager class

[OluRemiFour]

Close: #60

Implemented AccountManager with injected horizonClient, masterSecretKey, and network, plus the full public surface: generate(), create(), verify(), configureMultisig(), merge(), and fundTestnet(). The class is now a thin delegator in [src/accounts/index.ts], with the actual account operations in [src/accounts/operations.ts].

The operations use only the injected class-level Horizon client, add JSDoc on every public class method, and consistently wrap Stellar SDK/Horizon failures into typed SDK errors like ValidationError, AccountNotFoundError, HorizonSubmitError, and FriendbotError. I also exported the new API from [src/index.ts] and tightened type exports in [src/types/index.ts].

Verification passed with:

npm run type-check
npm test -- --runInBand
Delegation coverage was added in [tests/unit/accounts/account-manager.test.ts].

[OluRemiFour]

Done, Close: #63

[amina69]

@OluRemiFour you are suppose to push to develop not main. and was also suppose to branch of develop, it in the contributing.md...please read it before starting the project

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[OluRemiFour]

@OluRemiFour you are suppose to push to develop not main. and was also suppose to branch of develop, it in the contributing.md...please read it before starting the project

Fixed, Close: #63

[amina69]

@OluRemiFour test is failing