feat: move tailscale device discovery and settings into a plugin

Adds plugins/tailscale/ with a devices sidebar tab, admin settings
section, and re-auth dialog. Host Metrics tailscale manager card and
SSH re-auth banner parsing stay in core.

src/ui/sidebar/AppRail.tsx

@@ -32,6 +32,7 @@ export type RailView =
   | "automations"
   | "ai"
   | "fleets"
+  | "tailscale"
   | "workspaces"
   | "collab";