Fix outdated Usage tab hint text

[GoDiao]

The Usage tab in the web console has a hint that says:

"Aggregated from non-streaming requests only. Streaming calls are not counted yet."

This is no longer true — the gateway now records usage for both streaming and non-streaming requests. The hint should be removed or updated to reflect reality.

Files to change:

• src/web/templates/index.html — remove or update the <p class="hint"> line inside #usage section

Acceptance criteria:

• The hint text is removed or updated to accurately describe current behavior

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.