feat(gmail): add labels style command to change label colors

[iskw9973]

Summary

Adds a new gog gmail labels style command to change label background and text colors via Labels.Patch API.

Closes #456

Usage

gog gmail labels style <labelIdOrName> --background-color "#16a765" --text-color "#ffffff"

• Both --background-color and --text-color are required (Gmail API requires both)
• Colors must be from the predefined palette
• System labels are blocked, consistent with rename/delete
• Supports label resolution by name or ID (same pattern as rename/delete)
• Dry-run (--dry-run) and JSON output (--json) supported

Changes

• internal/cmd/gmail_labels.go — added GmailLabelsStyleCmd struct and Run method
• internal/cmd/gmail_labels_style_cmd_test.go — 6 test cases (exact ID, name fallback, system label blocked, not found, missing both colors, missing one color)
• README.md — added labels style to docs

Test plan

• make test — all packages pass
• Tested against live Gmail account with multiple color combinations