Support all Up/Down markets for price to beat and add time to resolution

[novatechflow]

Summary

This PR addresses two key improvements for Polymarket positions display:

1. Fix missing "Price to Beat" across Up/Down markets (server.go):

   • Generalized Up/Down market detection (isUpDown) across outcome ("Up"/"Down"), eventSlug, slug, and title (handling updown, up-or-down, up/down, up-down), rather than only matching btc-updown-.
   • Supports ETH, SOL, XRP, DOGE, hourly Bitcoin (bitcoin-up-or-down-*), and any other crypto interval / daily Up/Down markets.
   • Added slug field to Position struct.
   • Normalized condition IDs to lowercase (normID) and deduplicated query batches to avoid case mismatch and redundant Gamma queries.
   • Preserved previously cached PriceToBeat values across subsequent polls.

2. Add "Time to Resolution" in device's local timezone (index.html):

   • Added fmtResolution(endStr) to convert UTC timestamps (e.g. 2026-09-19T10:00:00Z) to the viewer's local device timezone.
   • Formats the resolution time (HH:MM for today, Tomorrow HH:MM, or MMM D HH:MM) alongside a countdown (<1m left, Xm left, Xh Ym left, Xd Yh left, or ended).
   • Renders a dedicated time to resolution row on position cards when endDate is present.
   • Prevents JavaScript UTC midnight timezone-shift pitfalls on date-only strings.

3. Tests & Docs (server_test.go, README.md):

   • Added TestFetchPositionsAddsAllUpDownMarketsPriceToBeat.
   • Added TestIndexShowsTimeToResolutionWhenAvailable.
   • Updated README.md.