fix(review): bugs, arch violations, design smells

P1 bugs:
- unix_launcher: shell_split respects quoted args (was split_whitespace)
- plugin-host: 5s timeout on external plugin search
- ui: handle engine init panic, wire error state
- ui-egui: read window config instead of always using defaults
- plugin-url: use OpenPath action instead of SpawnProcess+xdg-open

Architecture:
- remove WindowConfig (mirror of WindowCfg); use WindowCfg directly
- remove on_select closure from SearchResult (domain leakage)
- remove LaunchAction::Custom; add Plugin::on_selected + SearchEngine::on_selected
- apps: record frecency via on_selected instead of embedded closure

Design smells:
- frecency: extract decay_factor helper, write outside mutex
- apps: remove cfg(test) cache_path hack; add new_for_test ctor
- apps: stable ResultId using name+exec to prevent collision
- files: stable ResultId using full path instead of index
- plugin-host: remove k-launcher-os-bridge dep (WindowConfig gone)

crates/plugins/plugin-files/src/lib.rs

@@ -71,21 +71,19 @@ impl Plugin for FilesPlugin {
                     .unwrap_or(false)
             })
             .take(20)
-            .enumerate()
-            .map(|(i, entry)| {
+            .map(|entry| {
                 let full_path = entry.path();
                 let name = entry.file_name().to_string_lossy().to_string();
                 let is_dir = full_path.is_dir();
                 let title = if is_dir { format!("{name}/") } else { name };
                 let path_str = full_path.to_string_lossy().to_string();
                 SearchResult {
-                    id: ResultId::new(format!("file-{i}")),
+                    id: ResultId::new(&path_str),
                     title: ResultTitle::new(title),
                     description: Some(path_str.clone()),
                     icon: None,
                     score: Score::new(50),
                     action: LaunchAction::OpenPath(path_str),
-                    on_select: None,
                 }
             })
             .collect()