Resolves four bugs in the Kubernetes management interface:
1. **Cluster not found error** - commands/kube.rs::list_nodes (and all other
kube resource commands) look up clusters from state.clusters (in-memory map)
which was never populated from the kubeconfig_files table. Add a new
connect_cluster_from_kubeconfig Tauri command that reads the encrypted
kubeconfig from the DB, decrypts it, and inserts a ClusterClient into
state.clusters. Wire it into KubernetesPage on initial load and cluster
change so the in-memory map is always populated before any kube command runs.
2. **Dropdown selection has no effect** - same root cause as #1; activating a
kubeconfig only updated the DB flag but never loaded the client into memory.
handleClusterChange now calls connectClusterFromKubeconfigCmd after activation.
3. **GUID shown instead of cluster name** - ClusterOverview displayed the raw
internal UUID as the page subtitle. Now accepts a clusterName prop (populated
from kubeconfig.context) and renders that instead. ClusterDetails similarly
changed to show kubeconfig.context in the header, not the UUID.
4. **Bell icon not clickable** - Hotbar bell button had no onClick handler. Add
optional onNotifications / notificationCount props; badge count is now dynamic
rather than hardcoded. KubernetesPage wires up a notifications dialog showing
active cluster context and a link to the Events section.
All changes follow TDD: failing tests written first, then implementation.
