Add epoch range selector to PoVW Cycles charts

[zeroklaw]

Problem

Currently, PovwCycles.tsx hardcodes a chart display limit of 100 epochs:

const merged = mergedAll.length > 100 ? mergedAll.slice(-100) : mergedAll;

With the API soon returning all available epochs (not just 100), the frontend needs a proper UI for controlling how much data to display and analyze.

Proposed Solution

Add an epoch range selector to the PoVW Cycles page that allows users to:

1. Choose a display window — e.g., "Last 50", "Last 100", "Last 200", "All" — controlling how many epochs appear on the charts.
2. Adjust the PoVW rate lookback — the sidebar slider already goes up to max={100}, but once the API returns more data, this should be extended (or made dynamic based on available epoch count).

UI suggestions

• A dropdown or segmented control above/below the charts for quick range selection.
• Consider also enabling the lookback slider to go beyond 100 once more data is available.
• Summary stats (total grinding rewards, average % market) should continue to compute across the selected range, not just the displayed chart points.

Dependencies

• Depends on: API returning more than 100 epochs (removal of hardcoded cap).
• Can be developed in parallel with the API changes, using mock data.