Reapply thread storage fixes for 1.2.1

[cquxl]

Summary

Reapplies the thread storage fixes on top of the current Rust/Tauri 1.2.1 codebase.

• Respect CODEX_HOME when resolving the Codex local SQLite database path.
• Remove stale projected/sidebar thread rows when deletion reports Thread not found in local storage.
• Add an asset-level regression test so the injected renderer cleanup remains present.

Why

Profile/history merge workflows can leave sidebar rows pointing at stale projected thread IDs. The delete flow then reports Thread not found in local storage and leaves the row visible. Users running with a switched CODEX_HOME can also hit the wrong local storage database if the launcher falls back to the default home path.

Validation

• cargo test -p codex-plus-core --test assets
• npm run check
• cargo build --release -p codex-plus-launcher
• npm run vite:build
• npm run build
• Installed patched 1.2.1 binaries locally and verified codex-plus-plus.exe stayed running after launch smoke test.