rpc: make z_importkey generic over Chain

[oxarbitrage]

Fixes the main build, which broke after #400 (z_importkey/z_exportkey) merged.

#400's CI last ran 2026-06-08, before the "make the JSON-RPC layer generic over Chain" refactor landed on main (2026-06-19 … 06-22, #492). That refactor turned self.chain() into a generic C: Chain, but #400's import_key::call and its fetch_account_birthday helper still took a concrete FetchServiceSubscriber. The two were textually mergeable but type-incompatible, so once both were on main the build failed with:

error[E0308]: `?` operator has incompatible types
 --> zallet/src/components/json_rpc/methods.rs:1009:13
1009 |     self.chain().await?,
     expected `FetchServiceSubscriber`, found type parameter `C`

This migrates import_key::call and fetch_account_birthday to the generic Chain/ChainView API, mirroring the pattern the refactor applied to the other RPC methods (e.g. get_new_account): fetch_account_birthday now snapshots the chain and reads the treestate via ChainView::tree_state_as_of, building the birthday from the resulting ChainState.

Verified locally on current main: cargo build -p zallet, cargo clippy -p zallet --all-targets, cargo fmt --check, and cargo test -p zallet import_key all pass.

🤖 Generated with Claude Code

[nuttycom]

#506 seems like it does a better thing with the error types here; this isn't a database error. Otherwise this and #506 are functionally identical.

[oxarbitrage]

Closing in favor of #506