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/main.rs

@@ -14,8 +14,9 @@ use application::DriftwoodApplication;
 use config::{APP_ID, GSETTINGS_SCHEMA_DIR};
 
 fn main() -> ExitCode {
-    // Point GSettings at our compiled schema directory (dev builds)
-    std::env::set_var("GSETTINGS_SCHEMA_DIR", GSETTINGS_SCHEMA_DIR);
+    // Point GSettings at our compiled schema directory (dev builds).
+    // SAFETY: Called before any threads are spawned, at program start.
+    unsafe { std::env::set_var("GSETTINGS_SCHEMA_DIR", GSETTINGS_SCHEMA_DIR); }
 
     // Parse CLI arguments
     let parsed = cli::Cli::parse();