Add interactive splash screen (ASCII header + command menu)

[jdwit]

Summary

Add an interactive splash screen shown when ytstudio is run with no command (and optionally behind a ytstudio / ytstudio menu entrypoint). It gives the tool a recognizable header and turns the bare CLI into a friendly launcher: an ASCII logo + wordmark, a one-line tagline, the current auth/profile state, and an arrow-key menu of the top-level command groups with short descriptions.

This keeps the tool fully scriptable (every command stays available non-interactively) while giving humans a nicer entry point. AI-agent usage is unaffected: the splash only renders for an interactive TTY with no subcommand.

Proposed ASCII layout

True ASCII (no Unicode block/box glyphs) so it renders identically across terminals and fonts. Logo (play button) on the left, wordmark on the right, mirroring a typical TUI header:

  +-----------+    __   _______   ____ _____ _   _ ____ ___ ___
  |  |\       |    \ \ / /_   _| / ___|_   _| | | |  _ \_ _/ _ \
  |  | \      |     \ V /  | |   \___ \ | | | | | | | | | | | | |
  |  |  >     |      | |   | |    ___) || | | |_| | |_| | | |_| |
  |  | /      |      |_|   |_|   |____/ |_|  \___/|____/___\___/
  |  |/       |    manage & analyze your YouTube channel from the terminal
  +-----------+

Below the header: a status line, the prompt, and the selectable menu. The > marks the
highlighted row; the command name is bold, the description dimmed:

  Signed in as @channelhandle    |    profile: default

  What would you like to do?

> Videos        - bulk update, upload, search-replace metadata
  Analytics     - channel & video performance queries
  Comments      - list, reply, approve, reject, ban
  Livestreams   - schedule, start, stop, update broadcasts
  Playlists     - bulk-add by search, reorder by views
  Profile       - switch or manage channel profiles
  Status        - show auth & current configuration
  Quit

Selecting an item either runs the simple command directly (Status, Quit) or drops into that
group's own help/submenu (Videos, Analytics, ...). Menu items map 1:1 to the existing top-level
commands, so nothing new has to be wired into the command tree.

Not-yet-authenticated state

When there are no credentials/active profile, surface onboarding first instead of the channel menu:

  Not signed in.

  What would you like to do?

> Init          - set up Google OAuth credentials
  Login         - authenticate with YouTube
  Quit

Behavior / scope

• Render the splash only when: stdout is a TTY and no subcommand was given. Otherwise behave exactly as today (so pipes, CI, and agents are unaffected).
• Add a --no-splash / YTSTUDIO_NO_SPLASH=1 escape hatch, and keep ytstudio --help as the plain Typer help (unchanged).
• Menu selection: arrow keys + enter; also accept first-letter / number shortcuts. q or Ctrl-C quits cleanly.
• Colors via the existing Rich/terminal styling; degrade gracefully with NO_COLOR.
• The exact figlet wordmark + play-button glyphs above are a starting point; open to refinement (e.g. a tighter logo, or a "YouTube red" accent on the wordmark).

Open questions

• Standalone entrypoint: splash on bare ytstudio, or a dedicated ytstudio menu, or both?
• Should the status line do a (cached) channel-handle lookup, or stay purely local config to avoid an API call on startup?
• Wordmark text: YTSTUDIO (one word, as above) vs YT STUDIO (two words)?