Add follow toggles to tag chips on detail views

TagChip gains a followable mode that renders a small toggle inside the
chip: '+' follows the tag, '✓' (click to unfollow) once followed, and a
red marker with the reason as its title when e621 rejects it. Enabled on
the library detail's e621 metadata card (matched posts) and on online
post detail tags; guests see plain chips and the label click keeps
working beside the toggle.

frontend/src/components/TagChip.tsx

@@ -1,4 +1,5 @@
 import { cn } from "@/lib/cn";
+import { TagFollowToggle } from "@/features/follows/TagFollowToggle";
 
 const TAG_STYLES: Record<string, string> = {
   general:
@@ -26,11 +27,13 @@ export function TagChip({
   category,
   count,
   onClick,
+  followable = false,
 }: {
   tag: string;
   category?: string;
   count?: number;
   onClick?: () => void;
+  followable?: boolean;
 }) {
   const style = TAG_STYLES[category ?? "unknown"] ?? TAG_STYLES.unknown;
   const className = cn(
@@ -39,6 +42,21 @@ export function TagChip({
   );
   const label = count !== undefined ? `${tag} (${count})` : tag;
 
+  if (followable) {
+    return (
+      <span className={cn(className, "inline-flex items-center gap-1")}>
+        {onClick ? (
+          <button type="button" onClick={onClick} className="text-left">
+            {label}
+          </button>
+        ) : (
+          <span>{label}</span>
+        )}
+        <TagFollowToggle tag={tag} />
+      </span>
+    );
+  }
+
   if (!onClick) {
     return <span className={className}>{label}</span>;
   }