feat(app): add song detail page with chord chart and transposition

app/app/routes.ts

@@ -0,0 +1,6 @@
+import { type RouteConfig, index, route } from "@react-router/dev/routes";
+
+export default [
+  index("routes/home.tsx"),
+  route("songs/:id", "routes/songs.$id.tsx"),
+] satisfies RouteConfig;

app/app/routes/songs.$id.tsx

@@ -0,0 +1,70 @@
+import { useState } from "react";
+import { data } from "react-router";
+import type { Route } from "./+types/songs.$id";
+import { TransposeBar } from "~/components/transpose-bar";
+import { ChordChart } from "~/components/chord-chart";
+import { transposeSong } from "~/lib/transpose";
+import { getMockSong } from "~/lib/mock";
+import type { Song } from "~/lib/types";
+
+export function meta({ data }: Route.MetaArgs) {
+  if (!data) return [{ title: "Song not found" }];
+  return [
+    { title: `${data.song.meta.title} — PocketChords` },
+    { name: "description", content: `${data.song.meta.artist}` },
+  ];
+}
+
+export function loader({ params }: Route.LoaderArgs) {
+  const id = params.id ?? "";
+
+  // Temporary songs are stored in sessionStorage on the client.
+  // During SSR, we can't access sessionStorage — return a placeholder
+  // that the client will hydrate from sessionStorage.
+  if (id.startsWith("new-")) {
+    return { song: null as unknown as Song, tempId: id };
+  }
+
+  const song = getMockSong(id);
+  if (!song) throw data("Song not found", { status: 404 });
+  return { song, tempId: null };
+}
+
+export default function SongDetail({ loaderData }: Route.ComponentProps) {
+  const { tempId } = loaderData;
+
+  // Resolve song — either from loader or from sessionStorage (temp songs)
+  const [song] = useState<Song>(() => {
+    if (loaderData.song) return loaderData.song;
+    if (tempId && typeof window !== "undefined") {
+      const stored = sessionStorage.getItem(tempId);
+      if (stored) return JSON.parse(stored) as Song;
+    }
+    return loaderData.song;
+  });
+
+  const [offset, setOffset] = useState(0);
+
+  if (!song) {
+    return (
+      <div className="flex items-center justify-center h-dvh">
+        <p className="text-muted-foreground">Song not found.</p>
+      </div>
+    );
+  }
+
+  const displayed = transposeSong(song, offset);
+
+  return (
+    <div className="flex flex-col h-dvh max-w-lg mx-auto">
+      <TransposeBar
+        meta={song.meta}
+        offset={offset}
+        onOffsetChange={setOffset}
+      />
+      <div className="flex-1 overflow-y-auto">
+        <ChordChart sections={displayed.sections} />
+      </div>
+    </div>
+  );
+}