docs: add --theme launch example to TUI quickstart

[aheritier]

Summary

Adds a missing --theme launch example to the "Launching the TUI" quickstart code block in docs/features/tui/index.md.

The "Applying Themes" section and the CLI reference (docs/features/cli/index.md) both correctly document the --theme flag (added in #2933 and documented in #2936), but the quickstart block that lists other launch-time flags (--app-name, --sidebar, --disable-commands) omitted --theme, leaving the page internally inconsistent.

Documentation changes

File	Change
docs/features/tui/index.md	Added --theme dracula example to the "Launching the TUI" quickstart code block

Commits

Commit	Scope	Source PR
79f16572	Add --theme launch example to TUI quickstart	#2933

PRs audited

Source PR	Title	Doc status
#2933	feat: add --theme flag to preselect TUI theme	✅ Gap fixed (this PR)
#2932	docs: document --auth-token flag, OAuth callback security note, and TUI notification UX	✅ Already accurate and complete
#2936	docs: document --theme flag for docker agent run	✅ Already accurate and complete

[docker-agent]

Assessment: 🟢 APPROVE

This documentation change correctly adds the --theme launch example to the TUI quickstart. The flag name, example value (dracula is a valid built-in theme), comment wording, and placement are all accurate and consistent with the CLI reference and the Theming section. No issues found.