feat: production hardening (panic isolation, file logging, apps cache)

- Kernel::search wraps each plugin in catch_unwind; panics are logged and return []
- init_logging() adds daily rolling file at ~/.local/share/k-launcher/logs/
- AppsPlugin caches entries to ~/.cache/k-launcher/apps.bin via bincode; stale-while-revalidate on subsequent launches
- 57 tests pass

Cargo.toml

@@ -17,6 +17,7 @@ resolver = "2"
 
 [workspace.dependencies]
 async-trait = "0.1"
+bincode = { version = "2", features = ["serde"] }
 dirs = "6.0"
 futures = "0.3"
 iced = { version = "0.14", features = ["image", "svg", "tokio", "tiny-skia"] }