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/types/ui-types.ts

@@ -372,7 +372,8 @@ export type AdminSection =
   | "audit-log"
   | "ssl"
   | "plugins"
-  | "touch-input";
+  | "touch-input"
+  | "tailscale";
 export type ThemeId =
   | "dark"
   | "light"