Fix standalone release build (custom-protocol feature was never enabled)

cargo build --release alone doesn't produce a working standalone binary in
Tauri v2 — embedding the frontend vs. hitting devUrl at runtime is gated by
the tauri/custom-protocol Cargo feature, not the debug/release profile.
The scaffold never declared it, so even release builds tried to connect to
localhost:1420. Added the standard default-on custom-protocol feature
(cargo tauri dev strips it via --no-default-features for dev builds); a
plain `cargo build --release --locked` in src/ now produces a real
standalone binary with no dev-server dependency.

Also fixes vite.config.js's file-watch ignore pattern, left stale by the
src-tauri -> src rename: a bare "**/src/**" glob would have also matched
frontend/src itself, so it's now an absolute path scoped to the Rust
project dir specifically.

frontend/vite.config.js

@@ -1,9 +1,17 @@
+import { fileURLToPath } from "node:url";
+import path from "node:path";
 import { defineConfig } from "vite";
 import { sveltekit } from "@sveltejs/kit/vite";
 
 // @ts-expect-error process is a nodejs global
 const host = process.env.TAURI_DEV_HOST;
 
+// The Rust/Tauri project dir (../src relative to this file) — an absolute
+// path, not a bare "**/src/**" glob, because this repo has two directories
+// named `src`: this one (frontend/src, which must be watched for HMR) and
+// the sibling Rust crate (../src, which must not be).
+const rustProjectDir = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "../src");
+
 // https://vite.dev/config/
 export default defineConfig(async () => ({
   plugins: [sveltekit()],
@@ -25,8 +33,8 @@ export default defineConfig(async () => ({
         }
       : undefined,
     watch: {
-      // 3. tell Vite to ignore watching `src-tauri`
+      // 3. tell Vite to ignore watching the Tauri/Rust project dir
-      ignored: ["**/src-tauri/**"],
+      ignored: [`${rustProjectDir}/**`],
     },
   },
 }));

src/Cargo.toml

@@ -17,6 +17,15 @@ crate-type = ["staticlib", "cdylib", "rlib"]
 [build-dependencies]
 tauri-build = { version = "2", features = [] }
 
+[features]
+# Default-on so a plain `cargo build --release` produces a standalone binary
+# that embeds the built frontend instead of loading it from `devUrl` —
+# `cargo tauri dev` strips this back off via `--no-default-features` so dev
+# builds keep talking to the Vite dev server. See
+# https://v2.tauri.app/reference/cargo-features/#custom-protocol
+default = ["custom-protocol"]
+custom-protocol = ["tauri/custom-protocol"]
+
 [dependencies]
 tauri = { version = "2", features = ["protocol-asset"] }
 tauri-plugin-opener = "2"