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-cmd/src/lib.rs

@@ -36,7 +36,6 @@ impl Plugin for CmdPlugin {
             icon: None,
             score: Score::new(95),
             action: LaunchAction::SpawnInTerminal(cmd.to_string()),
-            on_select: None,
         }]
     }
 }