build: add build.rs to compile web dashboard during cargo build

Integrates the web dashboard build into the Rust build pipeline.
Runs npm install (if needed) and npm run build, with graceful
fallback warnings if Node.js tooling is unavailable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

build.rs

@@ -0,0 +1,47 @@
+use std::process::Command;
+
+fn main() {
+    // Re-run if web source files change
+    println!("cargo:rerun-if-changed=web/src/");
+    println!("cargo:rerun-if-changed=web/index.html");
+    println!("cargo:rerun-if-changed=web/package.json");
+    println!("cargo:rerun-if-changed=web/vite.config.ts");
+    println!("cargo:rerun-if-changed=web/tsconfig.json");
+
+    let web_dir = std::path::Path::new("web");
+
+    // Skip if node_modules not installed
+    if !web_dir.join("node_modules").exists() {
+        let status = Command::new("npm")
+            .args(["install"])
+            .current_dir(web_dir)
+            .status();
+
+        match status {
+            Ok(s) if s.success() => {}
+            Ok(s) => {
+                eprintln!("warning: `npm install` exited with {s}; web dashboard may be stale");
+                return;
+            }
+            Err(e) => {
+                eprintln!("warning: could not run `npm install`: {e}; web dashboard may be stale");
+                return;
+            }
+        }
+    }
+
+    let status = Command::new("npm")
+        .args(["run", "build"])
+        .current_dir(web_dir)
+        .status();
+
+    match status {
+        Ok(s) if s.success() => {}
+        Ok(s) => {
+            eprintln!("warning: `npm run build` exited with {s}; web dashboard may be stale");
+        }
+        Err(e) => {
+            eprintln!("warning: could not run `npm run build`: {e}; web dashboard may be stale");
+        }
+    }
+}