refactor(media): remove volume control from FE & BE

Volume sudah di-set default 0.3 di gateway (suara kecil saat play), dan
user bisa naikin sendiri di Discord (command media:volume) — jadi kontrol
volume lewat dashboard tak perlu. Hapus:
- BE: POST /api/media/volume route, mediaVolumeSchema, setVolume service
- FE: useMediaVolume hook, mediaApi.volume, slider volume di music-player
  + mini-player, field volume/setVolume di MediaPlayerProvider
Pertahankan COMMAND_MEDIA_VOLUME di gateway (masih dipakai command DC)
dan mic volume (terpisah, tetap di voice page).

services/frontend/src/components/media/mini-player.tsx

@@ -1,11 +1,10 @@
 "use client";
 
-import { Disc3, Music, SkipForward, Square, Volume2 } from "lucide-react";
+import { Disc3, Music, SkipForward, Square } from "lucide-react";
 import { useMediaPlayer } from "@/lib/hooks/use-media-player";
 
 export function MiniPlayer() {
-  const { playing, current, queue, volume, pending, skip, stop, setVolume } =
-    useMediaPlayer();
+  const { playing, current, queue, pending, skip, stop } = useMediaPlayer();
 
   // Nothing to show if no track is playing and nothing is queued
   if (!current && queue.length === 0) return null;
@@ -61,22 +60,6 @@ export function MiniPlayer() {
           </button>
         )}
       </div>
-
-      {/* Volume */}
-      <div className="flex items-center gap-2 shrink-0 ml-2">
-        <Volume2 className="size-3.5 text-text-secondary/60" />
-        <input
-          type="range"
-          min="0"
-          max="1"
-          step="0.05"
-          value={volume}
-          onChange={(e) => setVolume(Number(e.target.value))}
-          className="w-20 h-1 appearance-none rounded-full bg-glass-bg accent-primary cursor-pointer
-            [&::-webkit-slider-thumb]:appearance-none [&::-webkit-slider-thumb]:size-3 [&::-webkit-slider-thumb]:rounded-full [&::-webkit-slider-thumb]:bg-primary"
-          aria-label="Volume"
-        />
-      </div>
     </div>
   );
 }

services/frontend/src/components/media/music-player.tsx

@@ -1,24 +1,21 @@
 "use client";
 
-import { Disc3, Music, Play, SkipForward, Square, Volume2 } from "lucide-react";
+import { Disc3, Music, Play, SkipForward, Square } from "lucide-react";
 import Image from "next/image";
 import { useCallback, useState } from "react";
 
 import { Button } from "@/components/ui/button";
 import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/card";
 import { Input } from "@/components/ui/input";
-import { Slider } from "@/components/ui/slider";
 import {
   useMediaQueue,
   useMediaSkip,
   useMediaState,
   useMediaStop,
-  useMediaVolume,
   useMediaWsSync,
 } from "@/hooks";
-import type { WsHook } from "@/lib/ws-hook";
-
 import type { MediaState } from "@/lib/types";
+import type { WsHook } from "@/lib/ws-hook";
 
 interface MusicPlayerProps {
   ws: WsHook;
@@ -31,7 +28,6 @@ export function MusicPlayer({ ws, initialData }: MusicPlayerProps) {
   const queueMut = useMediaQueue();
   const skipMut = useMediaSkip();
   const stopMut = useMediaStop();
-  const volumeMut = useMediaVolume();
   const [queueUrl, setQueueUrl] = useState("");
   const [screenMode, setScreenMode] = useState(false);
 
@@ -47,14 +43,6 @@ export function MusicPlayer({ ws, initialData }: MusicPlayerProps) {
     setQueueUrl("");
   }, [queueUrl, queueMut, screenMode]);
 
-  const handleVolume = useCallback(
-    (value: number | readonly number[]) => {
-      const vol = Array.isArray(value) ? value[0] : value;
-      volumeMut.mutate(vol);
-    },
-    [volumeMut],
-  );
-
   return (
     <Card>
       <CardHeader>
@@ -143,18 +131,6 @@ export function MusicPlayer({ ws, initialData }: MusicPlayerProps) {
             <SkipForward className="size-4 mr-1" />
             Skip
           </Button>
-          <div className="flex items-center gap-2 ml-auto">
-            <Volume2 className="size-4 text-muted-foreground" />
-            <Slider
-              className="w-24"
-              defaultValue={[mediaState?.musicVolume ?? 0.3]}
-              value={[mediaState?.musicVolume ?? 0.3]}
-              onValueChange={handleVolume}
-              min={0}
-              max={1}
-              step={0.05}
-            />
-          </div>
         </div>
 
         {mediaState && mediaState.queue.length > 0 && (