fix(plugins): diagnostics crash minimales pour l'install de plugin (backend)

Écran noir hello-plugin (#120) récidivant après 3 fixes déjà mergés sans
capturer la cause réelle : on ne pouvait pas savoir si le crash venait de
l'install, du reconcile MCP ou d'un panic silencieux. Ajoute un panic hook
qui logge thread/location/backtrace, trace les étapes install/reconcile/
asset-protocol dans idea.log, et remplace les `.expect()` du superviseur MCP
plugin par une erreur typée au lieu d'un panic sur mutex empoisonné.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/app-tauri/src/lib.rs

@@ -47,6 +47,7 @@ use uuid::Uuid;
 use state::AppState;
 
 static EXIT_GUARD_CONFIRMED: AtomicBool = AtomicBool::new(false);
+static PANIC_HOOK_INSTALLED: AtomicBool = AtomicBool::new(false);
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 enum MainCloseAction {
@@ -69,6 +70,33 @@ fn should_install_exit_guard(window_label: &str) -> bool {
     window_label == "main"
 }
 
+fn install_panic_diagnostics_hook() {
+    if PANIC_HOOK_INSTALLED.swap(true, Ordering::SeqCst) {
+        return;
+    }
+    let previous = std::panic::take_hook();
+    std::panic::set_hook(Box::new(move |info| {
+        let message = info
+            .payload()
+            .downcast_ref::<&str>()
+            .copied()
+            .or_else(|| info.payload().downcast_ref::<String>().map(String::as_str))
+            .unwrap_or("<non-string panic payload>");
+        let location = info
+            .location()
+            .map(|l| format!("{}:{}:{}", l.file(), l.line(), l.column()))
+            .unwrap_or_else(|| "<unknown>".to_owned());
+        let thread = std::thread::current();
+        let thread_name = thread.name().unwrap_or("<unnamed>");
+        application::diag!("[panic] thread={thread_name} location={location} message={message}");
+        application::diag!(
+            "[panic] backtrace:\n{}",
+            std::backtrace::Backtrace::force_capture()
+        );
+        previous(info);
+    }));
+}
+
 fn apply_main_close_decision(
     guard: application::AppExitWorkGuardState,
     already_confirmed: bool,
@@ -151,6 +179,7 @@ pub fn run() {
             // click-launched AppImage (whose stderr is otherwise discarded). Best-effort:
             // if the file can't be opened the beacons simply stay on stderr.
             application::diag::set_log_path(app_data_dir.join("logs").join("idea.log"));
+            install_panic_diagnostics_hook();
             application::diag!("[startup] IdeA launched; diagnostics log armed");
             let app_state = AppState::build_with_resource_dir(app_data_dir, resource_dir);