feat(maestro): preflight pnpm + node at installer web step

The opt-in maestro-web step now checks for the frontend toolchain its
build.rs requires; missing tools prompt abort/skip (interactive) or skip
the refresh (non-interactive) instead of failing cargo with a raw panic.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/maestro-installer/src/main.rs

@@ -345,6 +345,39 @@ fn default_web_exec_start() -> Result<PathBuf, Box<dyn std::error::Error>> {
         .ok_or_else(|| "could not resolve $HOME (needed for web ExecStart)".into())
 }
 
+/// What to do at the (opt-in) web step given whether pnpm + node are present.
+#[derive(Debug, PartialEq, Eq)]
+enum WebPrereq {
+    /// Toolchain present — install maestro-web normally.
+    Ok,
+    /// Missing and we can ask: prompt the operator to abort or skip.
+    PromptMissing,
+    /// Missing and we can't prompt (non-interactive refresh): warn + skip.
+    SkipMissing,
+}
+
+fn classify_web_prereq(toolchain_present: bool, interactive: bool) -> WebPrereq {
+    if toolchain_present {
+        WebPrereq::Ok
+    } else if interactive {
+        WebPrereq::PromptMissing
+    } else {
+        WebPrereq::SkipMissing
+    }
+}
+
+/// True iff both `pnpm` and `node` respond to `--version` on PATH. maestro-web's
+/// build.rs needs them to build the embedded SPA bundle.
+fn frontend_toolchain_present() -> bool {
+    ["pnpm", "node"].iter().all(|tool| {
+        Command::new(tool)
+            .arg("--version")
+            .output()
+            .map(|o| o.status.success())
+            .unwrap_or(false)
+    })
+}
+
 /// Optional step: install `maestro-web` + its systemd user unit (NAP-249).
 /// Returns `true` iff the binary was installed (with or without systemd).
 fn handle_web(
@@ -390,6 +423,27 @@ fn handle_web(
         return Ok(false);
     }
 
+    // maestro-web's build.rs requires pnpm + node. Check before cargo install so
+    // a missing toolchain surfaces a clear choice instead of a raw build panic.
+    match classify_web_prereq(frontend_toolchain_present(), interactive) {
+        WebPrereq::Ok => {}
+        WebPrereq::PromptMissing => {
+            eprintln!("\nmaestro-web needs pnpm + node, which aren't installed.");
+            if confirm("Abort the whole install? (No = skip web UI)", false)? {
+                return Err("aborted: maestro-web prerequisites (pnpm + node) are missing".into());
+            }
+            eprintln!("Skipping maestro-web install.");
+            return Ok(false);
+        }
+        WebPrereq::SkipMissing => {
+            eprintln!(
+                "\nSkipping maestro-web refresh: pnpm + node not installed. \
+                 Existing unit left in place."
+            );
+            return Ok(false);
+        }
+    }
+
     // Install the binary. Even if cargo-install fails we keep going so the
     // operator can fix the cause and re-run; the existing daemon work
     // doesn't need to be undone.
@@ -514,3 +568,24 @@ fn print_summary(configured: bool, systemd_enabled: bool, web_installed: bool) {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn prereq_ok_when_toolchain_present() {
+        assert_eq!(classify_web_prereq(true, true), WebPrereq::Ok);
+        assert_eq!(classify_web_prereq(true, false), WebPrereq::Ok);
+    }
+
+    #[test]
+    fn prereq_prompts_when_missing_and_interactive() {
+        assert_eq!(classify_web_prereq(false, true), WebPrereq::PromptMissing);
+    }
+
+    #[test]
+    fn prereq_skips_when_missing_and_noninteractive() {
+        assert_eq!(classify_web_prereq(false, false), WebPrereq::SkipMissing);
+    }
+}