feat: Add watch-only addresses (#55)

[Sword-Smith]

Register an external viewing address, scoped to an account, to monitor its incoming UTXOs without any ability to spend. Supports ViewingAddress today (EC hybrid is scoped but gated on a serializable viewing-key form in neptune-wallet).

A single watch-only concept with an optional capability:

• Viewing key only: shows the total amount received (incoming UTXOs are confirmed against the block via the public receiver digest).
• Viewing key + receiver preimage (imported as a hex Digest, validated by preimage.hash() == receiver_digest): also detects spends and shows a real, timelock-aware balance (available vs locked, with the next unlock date).

Both announced UTXOs and guesser (mining) rewards are detected; guesser rewards are picked up from the block structure whenever a watched address is the block's guesser, mirroring par_scan_for_incoming_utxo.

Watch-only UTXOs live in their own per-account tables, so they never enter the account balance nor become spendable inputs, and they roll back with reorgs.

Backend: new wallet/watch_only.rs (parse/validate, scan, spend detection, balance breakdown), watch_only_addresses/watch_only_utxos tables + migrations, a scan hook in update_new_tip and rollback in roll_back, and three Tauri commands. Frontend: a new account-scoped /watch-only page with an import form and a per-address amount/balance table.

[codewordneptune]

I think this is a great and welcome feature that opens up many new use cases for the wallet app: checking a cold wallet balance safely, transparently monitoring donation/charity funds, just to name a few. Very inspired by where the wallet app is heading :)

A few considerations, mainly from the UX side:

• Scope: per account or global? Is per-account scope the best fit, or should watch addresses be global, like Contacts? A watched address isn't derived from the selected account, so the scoping can feel a bit arbitrary. Global would also make checking faster, since the user wouldn't need the right account selected first. Realistically, how many watch addresses is a typical user likely to have? I'd expect only a few, which to me argues for one global list rather than spreading them across accounts. I realize this might affect the storage design, so fine to treat this as a discussion point rather than a blocker.
• Navigation placement: if watch addresses end up global/not account-specific, I'd move them to the bottom section of the side nav, with the other non-account items. If not, then the current position is fine.
• Terminology: would "Name" fit better than "Label"? It would align with Accounts and Contacts, which both use a name.
• Required name/label: I'd make the name/label required rather than optional. It eases management in the long run when every watch address has a name.

Beyond these points, looks good to me!

I assume watch support for generation addresses is coming too (great!)
Neptune-Crypto/neptune-core#952

[Sword-Smith]

Agreed on point 3 and 4: Change "label" to "name", and make name required.

About the scope: Global or account-tied watch-only addresses?
I think, regardless of whether the watch-only addresses are linked to an account or not, there is a foot gun in the payout policy logic: Once the account that's supposed to do the payouts is no longer active (by the user switching account), the payouts will not be made, since only one account's secret key is unlocked at a time. With the global watch-only addresses, this fact, that the account doing the payouts is not active, might be easier to present to the user though, as the payout-policy icon can be made red or grey to indicate that it's not running. With the account-attached watch-only addresses, the watch-only address and its payout policy will simply not appear when the account is not active. On the other hand, the sync logic for global watch-only addresses might be complicated by the fact that the rest of the sync logic is tied to accounts. But I haven't pursued that hunch rigorously.

Also: A follow-up feature plays well together with associating the watch-only addresses with accounts:
#58

[Sword-Smith]

3 and 4 have been implemented: Rename "label" to "name" and make this value required.