fix(voice): screen share crashed gateway — Streamer never joined voice + unhandledRejection

Root cause: ScreenShareController created @dank074 Streamer but never called
streamer.joinVoiceChannel() — playStream threw 'Bot is not connected to a
voice channel', and since the code only used .finally() (no .catch), the
rejection became an unhandledRejection that took down the whole gateway
(graceful shutdown triggered, systemd restarted).

Fixes:
1. Resolve active channel + streamer.joinVoiceChannel(channel) before
   prepareStream/playStream (dank074 needs its OWN WebRTC voice connection).
2. .catch() on the playStream done promise — log + kill ffmpeg instead of
   crashing the process.
3. .catch() on playback.done in media.handler too.
4. stop() now kills ffmpeg AND stops the streamer's voice connection.

services/discord-gateway/src/modules/command-handler/media.handler.ts

@@ -178,12 +178,19 @@ export class MediaHandler {
           addedAt: Date.now(),
           status: "playing",
         };
-        playback.done.finally(() => {
-          this.screenPlayback = null;
-          if (currentTrackItem?.mode === "screen") {
-            currentTrackItem = null;
-          }
-        });
+        playback.done
+          .catch((err) => {
+            this.logger.error(
+              { error: err instanceof Error ? err.message : String(err) },
+              "Screen playback promise rejected",
+            );
+          })
+          .finally(() => {
+            this.screenPlayback = null;
+            if (currentTrackItem?.mode === "screen") {
+              currentTrackItem = null;
+            }
+          });
         this.logger.info({ url }, "Screen share started");
         return { id: cmd.id, success: true, data: buildStatusPayload() };
       } catch (err) {