feat(web): settings helptext behind a "?" popup instead of inline paragraphs

The Qobuz/Monitor/Discovery help I just added was always-visible and verbose.
Replace each with a compact "?" button (new reusable HelpButton) that opens the
same content in a popup (reuses the Modal), keeping the settings tabs clean.

web tsc + build clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/app/_ui/help-button.tsx

@@ -0,0 +1,25 @@
+"use client";
+
+import { useState, type ReactNode } from "react";
+import { Modal } from "./modal";
+
+/** A small "?" affordance that opens its help content in a popup, keeping settings pages
+ *  uncluttered. */
+export function HelpButton({ title, children }: { title: string; children: ReactNode }) {
+  const [open, setOpen] = useState(false);
+  return (
+    <>
+      <button
+        type="button"
+        className="help-btn"
+        aria-label={`Help: ${title}`}
+        onClick={() => setOpen(true)}
+      >
+        ?
+      </button>
+      <Modal open={open} onClose={() => setOpen(false)} title={title}>
+        <div className="help-body">{children}</div>
+      </Modal>
+    </>
+  );
+}