fix(arch): build with custom-protocol so the package can load its own UI

The PKGBUILD ran a bare `cargo build --release`. Without
`--features tauri/custom-protocol` Tauri does not embed the frontend and serves
it from devUrl instead, so the package compiled, linked, installed and passed
every check — then launched into "Could not connect to localhost: Connection
refused". `tauri build` passes that feature for you and the Android build passes
it explicitly; this path never did, so the Arch package has never worked.

Adds a check() that catches it at build time. It tests for the *assets*, not for
the dev URL: devUrl is part of the config blob generate_context!() embeds either
way, so grepping for it reports a failure on a correct build. A content-hashed
filename from the vite output can only appear if the bundle was embedded — which
is also why the fixed binary is ~400 KB larger.

Found by installing the package and launching it, which is the only thing that
would have found it.

packaging/arch/PKGBUILD

@@ -29,7 +29,45 @@ build() {
   bun run build
   # Only the raw binary is needed; packaging is done in package() below so we
   # control the Arch filesystem layout ourselves rather than via tauri-bundler.
-  (cd src-tauri && cargo build --release --locked)
+  #
+  # 🔴 `tauri/custom-protocol` is not optional. `tauri build` passes it for you;
+  # a bare `cargo build` does not, and without it Tauri loads the frontend from
+  # `devUrl` rather than the assets embedded from `frontendDist`. The result
+  # builds and installs cleanly and then cannot load its own UI. check() guards
+  # this.
+  (cd src-tauri && cargo build --release --locked --features tauri/custom-protocol)
+}
+
+check() {
+  cd "$_srcdir"
+
+  # A Tauri binary built without `custom-protocol` does not embed the frontend;
+  # it serves it from `devUrl` (http://localhost:1420) instead. It compiles,
+  # links and installs perfectly, then launches into "Could not connect to
+  # localhost: Connection refused" — which is what this package did for its
+  # entire existence, because `tauri build` adds that feature for you and a bare
+  # `cargo build` does not.
+  #
+  # Test for the *assets*, not for the dev URL: `devUrl` is part of the config
+  # blob that generate_context!() embeds either way, so its presence proves
+  # nothing. A content-hashed filename from the vite build can only be in the
+  # binary if the bundle was embedded — the with-feature binary is ~400 KB
+  # larger for exactly this reason.
+  local _binary="src-tauri/target/release/jellytau"
+  local _asset
+  _asset="$(basename "$(ls -1 build/_app/immutable/entry/*.js | head -n1)")"
+
+  if [ -z "$_asset" ]; then
+    echo "==> ERROR: no frontend build found — 'bun run build' did not produce build/_app." >&2
+    return 1
+  fi
+
+  if ! grep -qa "$_asset" "$_binary"; then
+    echo "==> ERROR: the frontend bundle is not embedded in the binary." >&2
+    echo "    Build with --features tauri/custom-protocol, or the packaged app" >&2
+    echo "    will start up unable to load its own UI." >&2
+    return 1
+  fi
 }
 
 package() {