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/k-launcher-kernel/Cargo.toml

@@ -8,3 +8,4 @@ async-trait = { workspace = true }
 futures = { workspace = true }
 serde = { workspace = true }
 tokio = { workspace = true }
+tracing = { workspace = true }