test: qualify account persistence and session expiry

[sarthakagrawal927]

Summary

• Add account-scoped SQLite persistence coverage and browser journey specifications for account switching and session expiry.
• Add a local-only expiry helper, guarded by non-production plus explicit test-auth mode and scoped to the caller session.
• Add four independent regressions proving the helper fails closed in production and for unauthorized callers.

Verification

• Independent review; existing full Vitest suite passed 511 tests.
• New expiry and persistence tests: 11 passed.
• TypeScript and Biome checks passed.
• Browser specifications are added but not executed in this pass.

Refs #14. Hosted Google OAuth, persistence and account-switch qualification remain open. No production deployment or migration.