refactor(equalizer): split Equalizer.tsx 508 → 166 LOC across 6 files (#680)

Extract the canvas frequency-response math, the custom vertical fader, the
AutoEQ parser, and the AutoEQ search panel out of Equalizer.tsx:

- utils/eqCurve.ts          biquadPeakResponse + drawCurve
- utils/autoEqParse.ts      AutoEq types + parseFixedBandEqString
- components/equalizer/VerticalFader.tsx
- hooks/useAutoEq.ts        AutoEQ search/apply state
- components/equalizer/AutoEqSection.tsx

Pure code-move, no behaviour change. Both call sites (AudioTab, PlayerBar)
default-import Equalizer unchanged.

src/hooks/useAutoEq.ts

@@ -0,0 +1,93 @@
+import { useState, useRef, useEffect } from 'react';
+import { useTranslation } from 'react-i18next';
+import { invoke } from '@tauri-apps/api/core';
+import { useEqStore } from '../store/eqStore';
+import { parseFixedBandEqString, type AutoEqVariant, type AutoEqResult } from '../utils/autoEqParse';
+
+/** AutoEQ search/apply state for the Equalizer. Loads the entries index lazily
+ * when the section opens, filters client-side, and applies fetched profiles via
+ * the EQ store. */
+export function useAutoEq() {
+  const { t } = useTranslation();
+  const applyAutoEq = useEqStore(s => s.applyAutoEq);
+
+  const [autoEqOpen, setAutoEqOpen] = useState(false);
+  const [autoEqQuery, setAutoEqQuery] = useState('');
+  const [autoEqResults, setAutoEqResults] = useState<AutoEqResult[]>([]);
+  const [autoEqLoading, setAutoEqLoading] = useState(false);
+  const [autoEqError, setAutoEqError] = useState<string | null>(null);
+  const [autoEqApplied, setAutoEqApplied] = useState<string | null>(null);
+  const [entriesLoading, setEntriesLoading] = useState(false);
+  const entriesCacheRef = useRef<Record<string, AutoEqVariant[]> | null>(null);
+
+  // AutoEQ: load entries index lazily when section opens, then filter client-side
+  async function ensureEntries() {
+    if (entriesCacheRef.current) return;
+    setEntriesLoading(true);
+    setAutoEqError(null);
+    try {
+      const json = await invoke<string>('autoeq_entries');
+      entriesCacheRef.current = JSON.parse(json);
+    } catch (e: unknown) {
+      setAutoEqError(e instanceof Error ? e.message : t('settings.eqAutoEqError'));
+    } finally {
+      setEntriesLoading(false);
+    }
+  }
+
+  useEffect(() => {
+    const q = autoEqQuery.trim().toLowerCase();
+    if (!entriesCacheRef.current || q.length < 1) { setAutoEqResults([]); return; }
+    const flat: AutoEqResult[] = [];
+    for (const [name, variants] of Object.entries(entriesCacheRef.current)) {
+      if (!name.toLowerCase().includes(q)) continue;
+      for (const v of variants) {
+        flat.push({ name, source: v.source, rig: v.rig, form: v.form });
+        if (flat.length >= 20) break;
+      }
+      if (flat.length >= 20) break;
+    }
+    setAutoEqResults(flat);
+  // entriesLoading in deps: re-runs after entries finish loading so a query typed
+  // during loading produces results immediately without needing a re-type.
+  }, [autoEqQuery, entriesLoading]);
+
+  async function applyAutoEqResult(result: AutoEqResult) {
+    setAutoEqLoading(true);
+    setAutoEqError(null);
+    try {
+      const text = await invoke<string>('autoeq_fetch_profile', {
+        name: result.name,
+        source: result.source,
+        rig: result.rig ?? null,
+        form: result.form,
+      });
+      if (!text) throw new Error(t('settings.eqAutoEqFetchError'));
+      const { gains: newGains, preamp } = parseFixedBandEqString(text);
+      applyAutoEq(result.name, newGains, preamp);
+      setAutoEqApplied(result.name);
+      setAutoEqQuery('');
+      setAutoEqResults([]);
+      setTimeout(() => setAutoEqApplied(null), 3000);
+    } catch (e: unknown) {
+      setAutoEqError(e instanceof Error ? e.message : t('settings.eqAutoEqFetchError'));
+    } finally {
+      setAutoEqLoading(false);
+    }
+  }
+
+  const toggleOpen = () => {
+    const opening = !autoEqOpen;
+    setAutoEqOpen(opening);
+    setAutoEqQuery('');
+    setAutoEqResults([]);
+    setAutoEqError(null);
+    if (opening) ensureEntries();
+  };
+
+  return {
+    autoEqOpen, autoEqQuery, autoEqResults, autoEqLoading, autoEqError, autoEqApplied,
+    entriesLoading, setAutoEqQuery, setAutoEqError, setAutoEqResults,
+    applyAutoEqResult, toggleOpen,
+  };
+}