Add Google Calendar integration

[klopez4212]

Summary

• Add a Google Calendar OAuth connection backed by desktop commands and keychain token storage.
• Surface real Google Calendar events in the sidebar and self profile status, with no seeded preview data.
• Show DM meeting icons from real user status text without exposing calendar details.

Snapshots

Sidebar calendar empty state

Google Calendar connection settings

Checks

• pnpm typecheck
• pnpm check:px-text
• pnpm exec biome check ...
• cargo check --manifest-path desktop/src-tauri/Cargo.toml --no-default-features --features system-keyring
• pre-push hooks: desktop-test, mobile-test, rust-tests, desktop-tauri-test