fix: strip leading whitespace before shebang in mcpedia-web wrapper script

The heredoc-based script writer in web.nix was indenting content inside
a nix '' string, which caused bash to prepend whitespace before the
#!/usr/bin/env bash shebang. The kernel rejected this as 'Exec format
error' (203/EXEC), preventing systemd from starting mcpedia-web.service.

Fix: use writeShellScriptBin instead of a raw heredoc. This ensures the
shebang starts at column 0 and uses the nix-resolved bash path.

web.nix

@@ -1,12 +1,6 @@
-# ─── Web: packages pre-built .next into a Nix store path ──────────────────
+# ─── Web: packages pre-built .next into a Nix store path ───
 # Next.js 16/Turbopack has a Bun incompatibility in the Nix sandbox,
 # so .next is built in CI via `bun run build` and packaged here.
-#
-# Usage:
-#   nix-build -E 'with import <nixpkgs> {}; callPackage ./web.nix { webNextDir = /abs/apps/web/.next; }'
-#
-# The wrapper symlinks .next from the Nix store (read-only, instant),
-# then runs `bunx next start` from the repo checkout.
 { pkgs
 , stdenv
 , writeShellScriptBin
@@ -15,6 +9,17 @@
 , ...
 }:
 
+let
+  mcpedia-web-script = writeShellScriptBin "mcpedia-web" ''
+    #!${pkgs.bash}/bin/bash
+    export NODE_ENV=production
+    export PORT=4016
+    cd /home/code/mcpedia/apps/web
+    rm -f .next
+    ln -s "${webNextDir}" .next
+    exec "${bun}/bin/bun" x next start -p 4016
+  '';
+in
 stdenv.mkDerivation {
   pname = "mcpedia-web";
   version = "1.0.0";
@@ -25,19 +30,7 @@ stdenv.mkDerivation {
     mkdir -p $out/share/mcpedia-web/.next
     cp -r "${webNextDir}/." $out/share/mcpedia-web/.next/
 
-    NEXT_DIR="$out/share/mcpedia-web/.next"
-    BUN_BIN="${bun}/bin/bun"
-
     mkdir -p $out/bin
-    cat > $out/bin/mcpedia-web <<WRAPPER
-      #!/usr/bin/env bash
-      export NODE_ENV=production
-      export PORT=4016
-      cd /home/code/mcpedia/apps/web
-      rm -f .next
-      ln -s "$NEXT_DIR" .next
-      exec "$BUN_BIN" x next start -p 4016
-    WRAPPER
-    chmod +x $out/bin/mcpedia-web
+    cp ${mcpedia-web-script}/bin/mcpedia-web $out/bin/mcpedia-web
   '';
 }