Detect missing WebView2 at startup, show download link

Checks Windows registry for WebView2 Runtime before Tauri
initializes. If missing, shows a native MessageBox explaining
the requirement and opens the Microsoft download page.

src-tauri/src/main.rs

@@ -2,5 +2,87 @@
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")]
 
 fn main() {
+    #[cfg(windows)]
+    if !webview2_check::is_webview2_installed() {
+        webview2_check::show_missing_dialog();
+        return;
+    }
+
     core_cooldown_lib::run()
 }
+
+#[cfg(windows)]
+mod webview2_check {
+    use std::ptr;
+    use winapi::shared::minwindef::{HKEY, MAX_PATH};
+    use winapi::um::winnt::{KEY_READ, REG_SZ};
+    use winapi::um::winreg::{RegOpenKeyExW, RegQueryValueExW, RegCloseKey, HKEY_LOCAL_MACHINE, HKEY_CURRENT_USER};
+    use winapi::um::winuser::{MessageBoxW, MB_OK, MB_ICONWARNING};
+    use winapi::um::shellapi::ShellExecuteW;
+
+    const WEBVIEW2_GUID: &str = r"SOFTWARE\WOW6432Node\Microsoft\EdgeUpdate\Clients\{F3017226-FE2A-4295-8BEB-E15AB5810CD5}";
+    const WEBVIEW2_GUID_USER: &str = r"Software\Microsoft\EdgeUpdate\Clients\{F3017226-FE2A-4295-8BEB-E15AB5810CD5}";
+
+    fn to_wide(s: &str) -> Vec<u16> {
+        s.encode_utf16().chain(std::iter::once(0)).collect()
+    }
+
+    fn check_registry_key(root: HKEY, subkey: &str) -> bool {
+        unsafe {
+            let subkey_w = to_wide(subkey);
+            let value_name = to_wide("pv");
+            let mut hkey: HKEY = ptr::null_mut();
+
+            let status = RegOpenKeyExW(root, subkey_w.as_ptr(), 0, KEY_READ, &mut hkey);
+            if status != 0 {
+                return false;
+            }
+
+            let mut data = [0u16; MAX_PATH];
+            let mut data_size = (data.len() * 2) as u32;
+            let mut data_type = 0u32;
+
+            let status = RegQueryValueExW(
+                hkey,
+                value_name.as_ptr(),
+                ptr::null_mut(),
+                &mut data_type,
+                data.as_mut_ptr() as *mut u8,
+                &mut data_size,
+            );
+
+            RegCloseKey(hkey);
+
+            if status != 0 || data_type != REG_SZ {
+                return false;
+            }
+
+            // Convert to string and check it's not empty or "0.0.0.0"
+            let len = (data_size as usize / 2).saturating_sub(1); // exclude null terminator
+            let version = String::from_utf16_lossy(&data[..len]);
+            !version.is_empty() && version != "0.0.0.0"
+        }
+    }
+
+    pub fn is_webview2_installed() -> bool {
+        check_registry_key(HKEY_LOCAL_MACHINE, WEBVIEW2_GUID)
+            || check_registry_key(HKEY_CURRENT_USER, WEBVIEW2_GUID_USER)
+    }
+
+    pub fn show_missing_dialog() {
+        let title = to_wide("Core Cooldown - WebView2 Required");
+        let message = to_wide(
+            "Microsoft WebView2 Runtime is required to run Core Cooldown, \
+             but it was not found on this system.\n\n\
+             Click OK to open the download page in your browser.\n\n\
+             After installing WebView2, restart Core Cooldown."
+        );
+        let url = to_wide("https://developer.microsoft.com/en-us/microsoft-edge/webview2/consumer");
+        let open = to_wide("open");
+
+        unsafe {
+            MessageBoxW(ptr::null_mut(), message.as_ptr(), title.as_ptr(), MB_OK | MB_ICONWARNING);
+            ShellExecuteW(ptr::null_mut(), open.as_ptr(), url.as_ptr(), ptr::null(), ptr::null(), 1);
+        }
+    }
+}