client/web: fix redirect logic when accessing login client over TS IP

Was previously failing to redirect to the manage client when accessing
the login client with the Tailscale IP.

Updates #10261
Fixes tailscale/corp#16348

Co-authored-by: Will Norris <will@tailscale.com>
Signed-off-by: Sonia Appasamy <sonia@tailscale.com>
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