Remove VS Code base64 transport and use stable study IDs

[c-bata]

Summary

• Send storage files from the VS Code extension host to the Webview as Uint8Array instead of base64-encoded strings.
• Remove the corresponding atob and binary-string conversion logic.
• Clean up the Webview message listener when the component unmounts.
• Change OptunaStorage.getStudy to use a study ID instead of a list index.
• Update SQLite and Journal lookups, routes, and Study links to use stable study IDs.
• Add a regression test covering Journal study IDs with deleted-study gaps.

The study detail route is now /study/:studyId.