client/web: button, link, and other small UI updates

Makes the following changes:
* Use “link” class in various spots
* Remove button appearance on Exit Node dropdown in readonly mode
* Update `-stone-` colors to `-gray-` (couple spots missed by
  original color config commit)
* Pull full ui/button component from admin panel, and update
  buttons throughout UI to use this component
* Remove various buttons in readonly view to match mocks
* Add route (and “pending approval”) highlights to Subnet router
  settings card
* Delete legacy client button styles from index.css
* Fix overflow of IPv6 address on device details view

Updates #10261

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

client/web/src/components/app.tsx

@@ -153,10 +153,7 @@ function Header({
         <LoginToggle node={node} auth={auth} newSession={newSession} />
       </div>
       {loc !== "/" && loc !== "/update" && (
-        <Link
-          to="/"
-          className="text-blue-500 font-medium leading-snug block mb-[10px]"
-        >
+        <Link to="/" className="link font-medium block mb-[10px]">
           &larr; Back to {node.DeviceName}
         </Link>
       )}