fix(bootstrap-installer): resolve powershell.exe by absolute path on Windows

The native Windows installer spawned PowerShell via the bare program name
`powershell.exe`, which trusts PATH to contain
%SystemRoot%\System32\WindowsPowerShell\v1.0. On machines whose PATH was
trimmed or truncated (Windows silently drops entries once the variable
exceeds its length limit), the lookup fails and the spawn dies with
"program not found" before install.ps1 runs at all — the installer then
stalls at "0 of 0 steps".

Resolve PowerShell by absolute path first (%SystemRoot%/%windir%), then
fall back to PATH (powershell 5.1, then pwsh 7), then a bare name as a
last resort. Also include the resolved interpreter in the spawn-failure
context; the old message printed only the script path, which misleadingly
read as if the .ps1 itself was missing.

apps/bootstrap-installer/src-tauri/src/powershell.rs

@@ -72,7 +72,7 @@ pub async fn run_script(
 
     let mut child: Child = cmd
         .spawn()
-        .with_context(|| format!("spawning {}", script_path.display()))?;
+        .with_context(|| format!("spawning {} via {}", script_path.display(), interpreter_label()))?;
 
     let stdout = child.stdout.take().expect("stdout was piped");
     let stderr = child.stderr.take().expect("stderr was piped");
@@ -177,8 +177,9 @@ async fn recv_cancel(rx: &mut Option<CancelRx>) {
 fn build_command(script_path: &Path, args: &[String]) -> Command {
     // We want PowerShell 5.1 / 7. install.ps1 uses 5.1-safe syntax everywhere.
     // Prefer `powershell.exe` (5.1 baseline, present on every Windows since 7)
-    // over `pwsh.exe` (7+, may not be present).
-    let mut cmd = Command::new("powershell.exe");
+    // over `pwsh.exe` (7+, may not be present). Resolve it by absolute path —
+    // see `windows_powershell_exe`.
+    let mut cmd = Command::new(windows_powershell_exe());
     cmd.arg("-NoProfile");
     cmd.arg("-ExecutionPolicy").arg("Bypass");
     cmd.arg("-File").arg(script_path);
@@ -200,6 +201,58 @@ fn build_command(script_path: &Path, args: &[String]) -> Command {
     cmd
 }
 
+/// Canonical PowerShell 5.1 location under a Windows root (`%SystemRoot%`).
+#[cfg(target_os = "windows")]
+fn powershell_under_root(root: &Path) -> std::path::PathBuf {
+    root.join("System32")
+        .join("WindowsPowerShell")
+        .join("v1.0")
+        .join("powershell.exe")
+}
+
+/// Resolves the PowerShell interpreter to spawn.
+///
+/// `Command::new("powershell.exe")` trusts PATH to contain
+/// `%SystemRoot%\System32\WindowsPowerShell\v1.0`. On machines whose PATH was
+/// trimmed or truncated (Windows silently drops entries once the variable grows
+/// past its length limit), that lookup fails and the spawn dies with
+/// "program not found" before install.ps1 ever runs — the installer then stalls
+/// at "0 of 0 steps". Resolve by absolute path first, then fall back to PATH
+/// (powershell 5.1, then pwsh 7), then a bare name as a last resort.
+#[cfg(target_os = "windows")]
+fn windows_powershell_exe() -> std::path::PathBuf {
+    for var in ["SystemRoot", "windir"] {
+        if let Ok(root) = std::env::var(var) {
+            let candidate = powershell_under_root(Path::new(&root));
+            if candidate.is_file() {
+                return candidate;
+            }
+        }
+    }
+
+    for exe in ["powershell.exe", "pwsh.exe"] {
+        if let Ok(found) = which::which(exe) {
+            return found;
+        }
+    }
+
+    std::path::PathBuf::from("powershell.exe")
+}
+
+/// Human-readable interpreter name for spawn-failure context. On Windows this
+/// is the resolved PowerShell path so a missing/odd interpreter is obvious in
+/// the log (the old message only printed the script path, which read as if the
+/// .ps1 itself was missing).
+#[cfg(target_os = "windows")]
+fn interpreter_label() -> String {
+    windows_powershell_exe().display().to_string()
+}
+
+#[cfg(not(target_os = "windows"))]
+fn interpreter_label() -> String {
+    "bash".to_string()
+}
+
 /// Parses the LAST line of stdout that looks like a JSON object matching
 /// the install.ps1 stage-result contract: `{ok: bool, stage: string, ...}`.
 ///