Harden Gmail errors with @lowerdeck helpers

[devdattatalele]

Aligns Gmail with the google-meet/google-analytics/youtube error pattern.

Adds gmail/src/lib/errors.ts with gmailServiceError, gmailApiError, gmailOAuthError. The Api/OAuth helpers use extractGoogleMessage to drill into data.error.errors[].reason, tag data.reason + data.upstreamStatus, and setParent(error) to preserve the upstream stack.

Replaces 20 raw throw new Error(...) across auth.ts + manage-draft / manage-labels / manage-settings / manage-thread with gmailServiceError. Wraps the OAuth callback / refresh / profile-lookup axios calls with try/catch + gmailOAuthError(operation, error) matching the youtube auth shape.

Behavior unchanged on the happy path. Biome clean.

[devdattatalele]

Hey @herber — friendly ping. This has been sitting green since June; happy to rebase or split if that helps land it. Same for #44. No worries if it's on the backlog — just letting you know I'm around to iterate.