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/k-launcher-plugin-host/src/lib.rs

@@ -105,7 +105,15 @@ impl Plugin for ExternalPlugin {
         }
 
         let result = match guard.as_mut() {
-            Some(io) => do_search(io, query).await,
+            Some(io) => tokio::time::timeout(
+                std::time::Duration::from_secs(5),
+                do_search(io, query),
+            )
+            .await
+            .unwrap_or_else(|_| {
+                tracing::warn!("plugin {} search timed out", self.name);
+                Err("timeout".into())
+            }),
             None => unreachable!(),
         };
 
@@ -125,7 +133,6 @@ impl Plugin for ExternalPlugin {
                         }
                         ExternalAction::OpenPath { path } => LaunchAction::OpenPath(path),
                     },
-                    on_select: None,
                 })
                 .collect(),
             Err(e) => {