feat(web): Follow button + clickable artist in album modals

AlbumModal showed the artist name as static text — no way to follow the artist
or jump to them. Adds a Follow affordance (todo #13):

- AlbumInfo gains optional `artistMbid`. When present, the modal title links the
  artist name to /discover/artist/{mbid} and shows a Follow/Following button that
  reflects live follow state.
- Follow state fetched via a new lightweight GET /api/artists?mbid= check
  (avoids pulling the full watched list + discography); Follow reuses
  POST /api/artists. Non-ok POST surfaces a toast (also chips at todo #14).
- Wired artistMbid in the three discovery contexts where following is valuable:
  Discover, Discover artist preview, and Last.fm. Library/discography callers
  pass none (already-followed artists) so the button stays hidden — graceful.

web 152 tests (the ?mbid= follow-state check asserted), tsc + build clean.

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

web/src/app/_ui/album-modal.tsx

@@ -3,6 +3,7 @@
 import { useEffect, useState, type ReactNode } from "react";
 import { Modal } from "./modal";
 import { CoverArt } from "./cover-art";
+import { toast } from "./toast";
 
 type Track = { position: number; title: string; lengthMs: number | null };
 
@@ -22,6 +23,8 @@ export type AlbumInfo = {
   artist: string;
   year?: string | null;
   type?: string | null;
+  /** When present, the modal shows a Follow button + links the artist name to their page. */
+  artistMbid?: string | null;
 };
 
 /** Reusable album modal: cover art + tracklist (fetched from MB by release-group MBID) with
@@ -40,6 +43,60 @@ export function AlbumModal({
   actions?: ReactNode;
 }) {
   const [tracks, setTracks] = useState<Track[] | "loading" | "error">("loading");
+  // null = unknown/loading; only meaningful when album.artistMbid is set.
+  const [followed, setFollowed] = useState<boolean | null>(null);
+
+  useEffect(() => {
+    if (!open || !album.artistMbid) {
+      setFollowed(false);
+      return;
+    }
+    let cancelled = false;
+    setFollowed(null);
+    fetch(`/api/artists?mbid=${encodeURIComponent(album.artistMbid)}`)
+      .then((r) => (r.ok ? r.json() : { followed: false }))
+      .then((d) => !cancelled && setFollowed(!!d.followed))
+      .catch(() => !cancelled && setFollowed(false));
+    return () => {
+      cancelled = true;
+    };
+  }, [open, album.artistMbid]);
+
+  async function follow() {
+    if (!album.artistMbid) return;
+    try {
+      const res = await fetch("/api/artists", {
+        method: "POST",
+        headers: { "content-type": "application/json" },
+        body: JSON.stringify({ mbid: album.artistMbid, name: album.artist }),
+      });
+      if (res.ok || res.status === 409) {
+        setFollowed(true);
+        toast(`Following ${album.artist}`);
+      } else {
+        toast(`Couldn't follow ${album.artist}`);
+      }
+    } catch {
+      toast(`Couldn't follow ${album.artist}`);
+    }
+  }
+
+  const titleNode: ReactNode = album.artistMbid ? (
+    <span className="modal-artist">
+      <a href={`/discover/artist/${album.artistMbid}`}>{album.artist}</a>
+      <button
+        type="button"
+        className="btn sm ghost"
+        onClick={follow}
+        disabled={followed !== false}
+        aria-label={followed ? "following" : "follow artist"}
+      >
+        {followed === null ? "…" : followed ? "Following" : "Follow"}
+      </button>
+    </span>
+  ) : (
+    album.artist
+  );
 
   useEffect(() => {
     if (!open) return;
@@ -69,7 +126,7 @@ export function AlbumModal({
   }, [open, album.rgMbid]);
 
   return (
-    <Modal open={open} onClose={onClose} title={album.artist} wide>
+    <Modal open={open} onClose={onClose} title={titleNode} wide>
       <div className="album-modal-head">
         <CoverArt rgMbid={album.rgMbid} alt={album.album} size={250} className="album-modal-cover" />
         <div>