Buttons on Now Playing and slash commands

[Isma-L154]

Closes #44

What changes for users

• Buttons on every Now Playing message: ⏮ ⏯ ⏭ ⏹ / 🔁 🔀 📜 🎤. Anyone in the bot's voice channel can use them; anyone else gets a private refusal. Pause and loop update the message in place; skip, previous, stop and shuffle post a line naming who pressed them; the queue is shown privately; the microphone posts live lyrics. When the next song starts, the previous message's buttons are greyed out.
• Every command also works as /command. /play suggests searches as you type, /loop and /effect offer their options, /volume enforces 0–100. Errors under / are visible only to whoever caused them. Aliases (!p, !bass…) stay !-only.
• !effect <name> / /effect name: applies an effect; !effect alone still shows the active one.

How it is built

• utils/controls.py: playback actions returning an Outcome, shared by commands and buttons so both say the same thing.
• utils/now_playing_view.py: the buttons. The lyrics button reaches the Lyrics cog through a provider the cog registers on load, so the view never imports a cog.
• utils/announcer.py: retires the previous controls. Retired views are stopped; a timeout=None view that is never stopped stays in discord.py's view store for the life of the process.
• /play defers first (voice + yt-dlp outlast Discord's 3s), and replies "Starting …" when Now Playing takes over, so the interaction is never left "thinking".
• /lyrics posts the live message to the channel, not as the interaction reply: an interaction reply can only be edited for 15 minutes, and the live message is edited for as long as the music plays.
• utils/errors.py unwraps HybridCommandError → app_commands.CommandInvokeError, and reports RangeError with its bounds.
• Slash commands are declared guild-only, and are registered by an owner-only, hidden !sync (cogs/admin.py) instead of at every start, because Discord rate-limits syncing and the bot restarts unattended.
• utils/context.hybrid_command: commands.hybrid_command under a signature mypy can check on cog methods (it cannot solve discord.py's own union of Concatenate types there).

Design: docs/design/2026-09-27-interactive-controls.md.

Testing

ruff, mypy and pytest: 577 passed, 1 skipped (POSIX-only), up from 506. New tests cover the controls, every button (including refusals and a destroyed player), retiring, the suggestion endpoint's failure modes, slash-specific replies and error unwrapping. The slash command payload was built and serialised locally: 20 commands, all guild-only, with the expected choices, range and autocomplete.

After deploying, send !sync once from the bot owner's account, then check in a server:

1. /play with a search: suggestions appear, "Starting …" answers, Now Playing shows both button rows.
2. Each button from inside the voice channel; any button from outside it (private refusal).
3. Skip: the old message's buttons turn grey and the new one's work.
4. ⏹, then press a button on that message: it is disabled.
5. /lyrics and the 🎤 button on a song with synced lyrics.
6. /effect with no option, then with nightcore; /volume 150 is rejected by Discord itself.