client/web: handle offline exit nodes

If the currently selected exit node is offline, render the exit node
selector in red with an error message. Update exit nodes in the dropdown
to indicate if they are offline, and don't allow them to be selected.

This also updates some older color values to use the new colors.

Updates #10261

Signed-off-by: Will Norris <will@tailscale.com>

client/web/src/hooks/exit-nodes.ts

@@ -222,8 +222,10 @@ export function trimDNSSuffix(s: string, tailnetDNSName: string): string {
   return s
 }
 
-export const noExitNode: ExitNode = { ID: "NONE", Name: "None" }
+// Neither of these are really "online", but setting this makes them selectable.
+export const noExitNode: ExitNode = { ID: "NONE", Name: "None", Online: true }
 export const runAsExitNode: ExitNode = {
   ID: "RUNNING",
   Name: "Run as exit node…",
+  Online: true,
 }