Use sink for already logged in output

[gtsiolis]

This will fix two issues with the login command:

1. Hide header/banner when already logged in - The login command now checks for existing auth (env var or stored token) before launching the TUI. If already logged in, it emits a colored note and exits without showing the header.
2. Note messages now have colors in plain output mode - Added styling to formatMessageEvent() so notes, warnings, and success messages render with colors in non-TTY mode (matching the TUI styling).

BEFORE	AFTER

[gtsiolis]

Cc @carole-lavillonniere in case this relates to #208 (comment).

[carole-lavillonniere]

issue: the convention so far is to not do any styling in non-TTY.
What about keeping the TUI in this case but just hide the header, as we do in a few places already:

lstk/internal/ui/run_awsconfig.go

Line 29 in 60b92a0

return runWithTUI(parentCtx, withoutHeader(), func(ctx context.Context, sink output.Sink) error {