Make video a per-server toggle instead of an env-wide setting

VIDEO_ENABLED is now permission rather than behaviour: it decides whether
the feature exists at all, while turning it on for a server is a toggle in
the panel or !video in chat, off by default. Video costs real CPU for
every playing channel, so that should be a deliberate choice rather than
something a config flag switches on everywhere.

With the env flag off the panel renders no toggle at all and !video says
so, and the switch applies from the next track — swapping tracks mid-play
would cut the current one.

The loop button no longer reads "выкл" either: it sat next to the video
button showing the same word, so the two states were indistinguishable.
Both now name what they do and rely on highlighting for state.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/src/App.tsx

@@ -183,6 +183,7 @@ export function App() {
               voiceChannels={voiceChannels}
               yourVoiceChannel={yourVoiceChannel}
               requireListener={me.features.requireListener}
+              videoAvailable={me.features.video}
               onAction={(action, payload) => void runAction(action, payload)}
               onSeek={(seconds) => void runAction("seek", { position: seconds })}
               onJoin={(channelId) => {