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/types.ts

@@ -26,6 +26,7 @@ export interface PlayerState {
   history: Track[];
   volume: number;
   loop: LoopMode;
+  videoEnabled: boolean;
   updatedAt: number;
 }
 
@@ -43,7 +44,7 @@ export interface ServerRef {
 export interface Me {
   user: { id: string; username: string };
   servers: ServerRef[];
-  features: { localLibrary: boolean; requireListener: boolean };
+  features: { localLibrary: boolean; requireListener: boolean; video: boolean };
 }
 
 export interface ServerStateResponse {