fix(desktop): show window before Keychain runtime initialization

[openai0229]

Summary

• create the configured Tauri window before opening the OS-backed vault
• initialize RuntimeHost asynchronously and gate every desktop command on readiness
• surface startup failures in a native dialog and exit non-zero after acknowledgement
• abort and join pending initialization before shutting down a ready runtime

Root cause

The signed macOS app waited in SecKeychainFindGenericPassword for authorization to an older cdhash-bound Keychain item before Tauri created its window or entered the event loop. This made an authorization wait look like a windowless launch hang.

Verification

• cargo check -p chat2db-desktop --all-targets
• cargo test -p chat2db-desktop --lib (37 passed)
• cargo clippy -p chat2db-desktop --all-targets -- -D warnings
• git diff --check