TW-4910: TUI scope mismatch — structured errors, flash safety, q-to-quit

[qasim-nylas]

Summary

• Surface insufficient_scopes API errors with actionable suggestions (nylas auth show, nylas auth login, nylas auth switch) instead of generic error messages; falls back to human-readable text when the API message is empty
• Add goroutine-safe FlashLoadError helper to prevent data races when TUI views flash errors from background Load() goroutines
• Add q/Q to quit (k9s/less/top convention), guarded in filter mode, command palette, and detail views (compose/forms)
• Improve APIError.Error() to surface both message and type for better diagnostics
• Add structured 429 rate-limit and 5xx server error classification in WrapError

Test plan

• go test ./internal/domain/ — APIError.Error() all branches + Unwrap
• go test ./internal/cli/common/ — insufficient_scopes with/empty message, generic 403 fallthrough, 429, 500
• go test ./internal/tui/ -run TestFlash — deadlock-freedom, pre-run flash visibility
• go test ./internal/tui/ -run TestAppQKey — q/Q stop behavior
• Manual: nylas tui → verify q quits, but not during : command mode, / filter, or compose forms

Docs

• nylas tui

[AaronDDM]

LGTM 👍