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/orphan.rs

@@ -14,14 +14,12 @@ pub struct CleanupSummary {
 }
 
 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")
 }