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/tests/sidebar/rail-items.test.ts

@@ -57,6 +57,7 @@ describe("RAIL_ITEMS", () => {
       "snippets",
       "macros",
       "fleets",
+      "tailscale",
       "automations",
       "ai",
       "history",