faucet: validate address and show readable dispense errors

[Gmin2]

two small UX fixes on the faucet form.

1. address sanity check before sending. an empty or obviously typo'd address (e.g. "notanaddress") used to be sent to the node and come back as a raw 500. now it is caught client-side with an inline message. the check is deliberately loose (bech32 / base58 shape only, no checksum or network), the node stays the source of truth, so it never rejects a valid address.

2. readable dispense errors. on failure the form showed Failed to dispense coins: [ConnectError: [internal] dispense coins: -5: ...]. now the connect error is mapped to a short message for the common cases (invalid address, amount out of range, faucet out of funds / rate-limited) with a generic fallback.

verified: tsc, biome and prettier clean; helper logic checked against the actual signet error responses (invalid address, wrong-network address, over-limit amount) and against valid/invalid address samples; page renders against signet.

[octobocto]

It's possible to extract the message from connect! Not sure of the exact field, but I think it's ConnectError.from(err).message.

For dispenseErrorMessage, set the correct errors directly in the backend instead! Converting the backend error messages is brittle.

[Gmin2]

thanks, in 7f127f3 dropped dispenseErrorMessage (and the client-side regex) entirely, the frontend now just shows ConnectError.from(err).rawMessage. used rawMessage rather than message so the ui does not get the [code] prefix and also fix the the invalid-address case was falling through to CodeInternal with the raw core string. it now returns a clean CodeInvalidArgument