client/web: render 404 message in empty card

Switch the "feature disabled" page to use the same treatment.

Updates #10261

Signed-off-by: Will Norris <will@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f