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/locales/en.json

@@ -737,6 +737,7 @@
     "snippets": "Snippets",
     "macros": "Macros",
     "fleets": "Fleets",
+    "tailscale": "Tailscale",
     "automations": "Automations",
     "workspaces": "Workspaces",
     "hostManager": "Host Manager",
@@ -3507,6 +3508,7 @@
     "sectionApiKeys": "API Keys",
     "sectionAuditLog": "Audit Log",
     "sectionSsl": "SSL / Let's Encrypt",
+    "sectionTailscale": "Tailscale",
     "sectionTouchInput": "Touch Input",
     "touchEnabled": "Enable Touch Input",
     "touchEnabledDesc": "Allow one-finger touch scrolling in terminals",