Add launch crash detection with detailed error dialog, fix all warnings

Detect AppImages that crash immediately after spawning (within 1.5s) by
capturing stderr and using try_wait(). Show a full AlertDialog with a
plain-text explanation, scrollable error output, and a copy-to-clipboard
button. Covers Qt plugin errors, missing libraries, segfaults, permission
issues, and display connection failures.

Move launch operations to background threads in both the detail view and
context menu to avoid blocking the UI during the 1.5s crash detection
window.

Suppress all 57 compiler warnings across future-use modules (backup,
notification, report, watcher) and individual unused fields/variants in
other core modules.

src/core/updater.rs

@@ -390,6 +390,7 @@ fn extract_update_info_runtime(path: &Path) -> Option<String> {
 // -- GitHub/GitLab API types for JSON deserialization --
 
 #[derive(Deserialize)]
+#[allow(dead_code)]
 struct GhRelease {
     tag_name: String,
     name: Option<String>,
@@ -405,6 +406,7 @@ struct GhAsset {
 }
 
 #[derive(Deserialize)]
+#[allow(dead_code)]
 struct GlRelease {
     tag_name: String,
     name: Option<String>,