feat: add band_extract audio source type for frequency band filtering

New audio source type that filters a parent source to a specific frequency
band (bass 20-250Hz, mid 250-4kHz, treble 4k-20kHz, or custom range).
Supports chaining with frequency range intersection and cycle detection.
Band filtering applied in both CSS audio streams and test WebSocket.

server/src/wled_controller/static/locales/zh.json

@@ -1248,12 +1248,15 @@
     "audio_source.title": "音频源",
     "audio_source.group.multichannel": "多声道",
     "audio_source.group.mono": "单声道",
+    "audio_source.group.band_extract": "频段提取",
     "audio_source.add": "添加音频源",
     "audio_source.add.multichannel": "添加多声道源",
     "audio_source.add.mono": "添加单声道源",
+    "audio_source.add.band_extract": "添加频段提取源",
     "audio_source.edit": "编辑音频源",
     "audio_source.edit.multichannel": "编辑多声道源",
     "audio_source.edit.mono": "编辑单声道源",
+    "audio_source.edit.band_extract": "编辑频段提取源",
     "audio_source.name": "名称：",
     "audio_source.name.placeholder": "系统音频",
     "audio_source.name.hint": "此音频源的描述性名称",
@@ -1281,6 +1284,17 @@
     "audio_source.error.name_required": "请输入名称",
     "audio_source.audio_template": "音频模板：",
     "audio_source.audio_template.hint": "定义此设备使用哪个引擎和设置的音频采集模板",
+    "audio_source.band_parent": "父音频源：",
+    "audio_source.band_parent.hint": "要从中提取频段的音频源",
+    "audio_source.band": "频段：",
+    "audio_source.band.hint": "选择频段预设或自定义范围",
+    "audio_source.band.bass": "低音 (20–250 Hz)",
+    "audio_source.band.mid": "中音 (250–4000 Hz)",
+    "audio_source.band.treble": "高音 (4000–20000 Hz)",
+    "audio_source.band.custom": "自定义范围",
+    "audio_source.freq_low": "低频 (Hz)：",
+    "audio_source.freq_high": "高频 (Hz)：",
+    "audio_source.freq_range": "频率范围",
     "audio_source.test": "测试",
     "audio_source.test.title": "测试音频源",
     "audio_source.test.rms": "RMS",