Clarify that 'Estimated Usage' reflects transcoding only, not AI compute

[rickstaa]

Problem

The homepage "Estimated Usage" chart displays network usage in minutes — a metric inherited from Livepeer's transcoding origins to which does an estimation of minutes based on fees fee_derived_minutes = volume_eth / (average_price_per_pixel × average_pixel_per_minute). As the network has shifted toward AI, this figure no longer reflects actual network activity:

• AI compute jobs are billed per second of compute.

Raised by Authority_Null in Discord.

Proposal

Either:

• Replace the chart with a metric that makes sense for the current network (e.g. AI compute-seconds, LLM tokens, or a combined view), or
• Remove the chart entirely until a meaningful replacement exists.

Lives in pages/index.tsx (~line 198-233).