feat(init): prompt for + persist a Discord username during setup

[barnabasbusa]

Summary

Stacked on top of #105. Adds an optional Discord username step to `panda init` so build-completion DMs work without ever having to remember the `--discord-dm` flag.

• Interactive: after authentication, prompts `Discord username:` on a TTY. Blank = skip.
• Non-interactive: `--discord-username @barnabasbusa` sets it directly; `--skip-discord` silences the prompt entirely. No TTY ⇒ no prompt and no failure (CI-safe).
• Idempotent: if `notifications.discord.username` is already set in `config.user.yaml`, prints `already configured for …` and moves on.
• Persistence: same `config.SaveUserConfig` path that `panda build`'s first-use save uses in feat(build): DM the user on Discord when a triggered build finishes #105, so both flows produce identical YAML.

Why

You asked for the username to be remembered after setup so `panda build geth` notifies you without flag plumbing. #105 already auto-saves on first `--discord-dm` use; this PR also gives you a chance to set it up front during `panda init`.

Stack

Base: `bbusa/discord-dm-on-build` (PR #105). Please merge #105 first; this PR will then retarget to `master` automatically.

Test plan

• Fresh `panda init` on a TTY: prompts after auth, accepts `barnabasbusa`, writes to `config.user.yaml`.
• `panda init --discord-username '@bbusa' --skip-auth --skip-docker --skip-start --force`: no prompt, value persisted.
• `panda init --skip-discord --skip-auth --skip-docker --skip-start --force`: no prompt, no save.
• `panda init` with stdin not a TTY (e.g. `</dev/null`): no prompt, no error.
• Re-run `panda init` after a value is configured: prints `already configured for …`, no re-prompt.
• After init, `panda build geth` (no flag) → DM still goes out because the default is wired.