fix(auth): update config file when switching default grant

[mqasimca]

Summary

• Fixed a bug where nylas auth switch only updated the default grant in the keyring but not in the config file (~/.config/nylas/config.yaml)
• The config file's default_grant field remained stale after switching accounts
• Now both SwitchGrant and SwitchGrantByEmail update the keyring (authoritative source) and the config file

Problem

When running nylas auth switch <email>:

1. nylas auth list showed the correct default (reads from keyring) ✓
2. ~/.config/nylas/config.yaml still showed the old default_grant value ✗

This inconsistency caused confusion when inspecting the config file directly.

Solution

Added a setDefaultGrant() helper method in GrantService that updates both:

1. Keyring via grantStore.SetDefaultGrant() - authoritative source
2. Config file via config.Load() → set DefaultGrant → config.Save()

Config save errors are non-fatal since the keyring is the source of truth.

Changes

• internal/app/auth/grants.go: Added setDefaultGrant() helper, updated SwitchGrant() and SwitchGrantByEmail() to use it
• internal/app/auth/grants_test.go: Added comprehensive unit tests for GrantService switch operations

Test plan

• Unit tests pass (go test ./internal/app/auth/...)
• make ci passes (fmt, vet, lint, tests, security, vuln)
• Manual test: nylas auth switch <email> updates both keyring and config file