refactor(watch-only): replace unwraps with typed errors and logging

[alienx5499]

Description and Notes

This PR improves error handling in crates/floresta-watch-only by removing panic-prone unwrap/expect paths and replacing them with typed errors, guarded handling, and contextual logging.

Main changes:

• Add thiserror based WatchOnlyError and KvDatabaseError
• Add WatchOnlyError::Descriptor(String) and propagate descriptor parsing/derivation errors
• Replace panic paths in block/mempool/cache logic with safe Option handling and error! logs
• Replace several DB layer unwraps with safe handling in kv_database
• Add unit test for invalid descriptor error path

Related issues: #463, #86

How to verify the changes you have done?

From repo root:

cargo +nightly fmt -p floresta-watch-only
cargo test -p floresta-watch-only
cargo clippy -p floresta-watch-only --all-targets --all-features -- -D warnings

Note: Overlaps #931 slightly in maybe_derive_addresses (lib.rs); easy resolve after #931 is on master.