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

crates/plugins/plugin-apps/Cargo.toml

@@ -9,6 +9,8 @@ path = "src/lib.rs"
 
 [dependencies]
 async-trait = { workspace = true }
+bincode = { workspace = true }
+dirs = { workspace = true }
 k-launcher-kernel = { path = "../../k-launcher-kernel" }
 nucleo-matcher = "0.3"
 serde = { workspace = true }