feat: SETUP.EXE detection + separate setup .jsdos bundle

When a game archive contains SETUP.EXE, INSTALL.EXE, CONFIG.EXE or
CUSTOM.EXE (or .com/.bat variants), the upload now:

- Detects and stores the path in game metadata (setup_exe / has_setup)
- Generates a second .jsdos bundle with SETUP.EXE as autoexec:
  - Standard mode: {gameId}.setup.jsdos (full game + setup autoexec)
  - Sockdrive mode: .jsdos-setup/ directory alongside .jsdos/
- Both bundles share the same game files; only the autoexec differs

Frontend changes:
- Router strips query params from hash so ?setup=1 doesn't leak into id
- GameDetail shows a "Setup" button between Play and Edit (only when
  game.has_setup is true), navigating to /play/{id}?setup=1
- Play.svelte detects ?setup=1, loads setupBundleUrl() instead, and
  shows setup-specific overlay text

Persistence is handled automatically by js-dos v8 via the built-in
auto-save mechanism (ci.persist() → browser OPFS), so any config
changes made via SETUP.EXE survive across sessions.

frontend/src/pages/Play.svelte

@@ -1,9 +1,12 @@
 <script>
-  import { getGame, bundleUrl } from "../lib/api.js";
+  import { getGame, bundleUrl, setupBundleUrl } from "../lib/api.js";
   import { push } from "../lib/router.js";
 
   let { id } = $props();
 
+  // Read query params from the hash — window.location is available after mount
+  let isSetup = $state(false);
+
   let game = $state(null);
   let loading = $state(true);
   let error = $state("");
@@ -16,6 +19,8 @@
 
   async function load() {
     loading = true;
+    // Detect ?setup=1 from the hash-based URL
+    isSetup = window.location.hash.includes("setup=1");
     try {
       game = await getGame(id);
     } catch (e) {
@@ -54,7 +59,7 @@
 
     // Start the DOS emulator
     try {
-      const url = bundleUrl(game);
+      const url = isSetup ? setupBundleUrl(game) : bundleUrl(game);
       if (!url || !window.Dos) throw new Error("Failed to resolve game bundle");
       dosCI = await window.Dos(dosContainer, { url });
       running = true;
@@ -71,6 +76,10 @@
     window.location.reload();
   }
 
+  function goBack() {
+    push(`/game/${id}`);
+  }
+
   $effect(() => { load(); });
 
   // Auto-start when both game data and the container exist
@@ -98,7 +107,9 @@
   <!-- Top bar: shown when game is loaded -->
   <div class="play-header" class:active={game}>
     {#if game}
-      <button class="link-button" onclick={stopEmulator}>← {game.title}</button>
+      <button class="link-button" onclick={stopEmulator}>
+        ← {isSetup ? "Back" : game.title}
+      </button>
       {#if running}
         <button class="btn stop-btn" onclick={stopEmulator}>⏹ Stop</button>
       {/if}
@@ -119,8 +130,13 @@
       {:else if booting}
         <div class="overlay-icon">⚙️</div>
         <h2>{game?.title || ""}</h2>
-        <p>Starting emulator...</p>
-        <p class="save-hint">💾 Game saves are stored automatically</p>
+        {#if isSetup}
+          <p>Starting setup utility...</p>
+          <p class="save-hint">⚙️ Configure controls, then exit SETUP to save</p>
+        {:else}
+          <p>Starting emulator...</p>
+          <p class="save-hint">💾 Game saves are stored automatically</p>
+        {/if}
       {:else if error}
         <div class="overlay-icon">⚠️</div>
         <h2>Failed to start</h2>