Validate Stellar DID/address format and remove hardcoded default recipient

[Kingvic300]

Summary

• Adds isStellarAddress / isStellarTestnetDid validators in src/lib/validation.ts.
• Removes the hardcoded default subjectDid Stellar address from the credential form's initial state (still available via the existing "Fill Example" button and input placeholder).
• Rejects malformed Subject DIDs in handleCreate with a clear toast error before reaching the SDK.
• makeDidForAddress now validates the address and returns null instead of building a malformed DID.
• Adds inline red helper text and aria-invalid on the Subject DID input when the value doesn't match the expected format.

Fixes #11.

Test plan

• Submit with an empty Subject DID → blocked with existing "fill all fields" message.
• Submit with did:pkh:stellar:testnet:invalid → blocked with new "Invalid Subject DID format" message.
• Submit with a valid testnet DID → proceeds to SDK call.
• Confirm the Subject DID field is empty by default (no longer pre-filled with a real address).

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