Fix second audit findings and restore crash detection dialog

Address 29 issues found in comprehensive API/spec audit:
- Fix .desktop Exec key path escaping per Desktop Entry spec
- Fix update dialog double-dispatch with connect_response
- Fix version comparison total ordering with lexicographic fallback
- Use RETURNING id for reliable upsert in database
- Replace tilde-based path fallbacks with proper XDG helpers
- Fix backup create/restore path asymmetry for non-home paths
- HTML-escape severity class in security reports
- Use AppStream <custom> element instead of <metadata>
- Fix has_appimage_update_tool to check .is_ok() not .success()
- Use ListBoxRow instead of ActionRow::set_child in ExpanderRow
- Add ELF magic validation to architecture detection
- Add timeout to extract_update_info_runtime
- Skip symlinks in dir_size calculation
- Use Condvar instead of busy-wait in analysis thread pool
- Restore crash detection to single blocking call architecture

src/core/integrator.rs

@@ -25,20 +25,34 @@ impl From<std::io::Error> for IntegrationError {
     }
 }
 
+/// Escape a string for use inside a double-quoted Exec argument in a .desktop file.
+/// Per the Desktop Entry spec, `\`, `"`, `` ` ``, and `$` must be escaped with `\`.
+fn escape_exec_arg(s: &str) -> String {
+    let mut out = String::with_capacity(s.len());
+    for c in s.chars() {
+        match c {
+            '\\' | '"' | '`' | '$' => {
+                out.push('\\');
+                out.push(c);
+            }
+            _ => out.push(c),
+        }
+    }
+    out
+}
+
 pub struct IntegrationResult {
     pub desktop_file_path: PathBuf,
     pub icon_install_path: Option<PathBuf>,
 }
 
 fn applications_dir() -> PathBuf {
-    dirs::data_dir()
-        .unwrap_or_else(|| PathBuf::from("~/.local/share"))
+    crate::config::data_dir_fallback()
         .join("applications")
 }
 
 fn icons_dir() -> PathBuf {
-    dirs::data_dir()
-        .unwrap_or_else(|| PathBuf::from("~/.local/share"))
+    crate::config::data_dir_fallback()
         .join("icons/hicolor")
 }
 
@@ -90,21 +104,22 @@ pub fn integrate(record: &AppImageRecord) -> Result<IntegrationResult, Integrati
 
     let icon_id = format!("driftwood-{}", app_id);
 
-    let desktop_content = format!(
-        "[Desktop Entry]\n\
-         Type=Application\n\
-         Name={name}\n\
-         Exec=\"{exec}\" %U\n\
-         Icon={icon}\n\
-         Categories={categories}\n\
-         Comment={comment}\n\
-         Terminal=false\n\
-         X-AppImage-Path={path}\n\
-         X-AppImage-Version={version}\n\
-         X-AppImage-Managed-By=Driftwood\n\
-         X-AppImage-Integrated-Date={date}\n",
+    let desktop_content = format!("\
+[Desktop Entry]
+Type=Application
+Name={name}
+Exec=\"{exec}\" %U
+Icon={icon}
+Categories={categories}
+Comment={comment}
+Terminal=false
+X-AppImage-Path={path}
+X-AppImage-Version={version}
+X-AppImage-Managed-By=Driftwood
+X-AppImage-Integrated-Date={date}
+",
         name = app_name,
-        exec = record.path,
+        exec = escape_exec_arg(&record.path),
         icon = icon_id,
         categories = categories,
         comment = comment,