feat(level): chart XP over time in /level check

[CommunityPoke]

Summary

/level check now shows a chart of XP over time, addressing the feature request for a visible XP history graph.

There was no historical XP data — each levels doc only stored the current xp total. This PR adds a bounded xpByDay map ('YYYY-MM-DD' → cumulative total XP, UTC) written in the existing awardXP set(..., { merge: true }) call, so history accumulates at one field per day with no extra writes.

/level check renders the last 30 days as a line chart PNG and attaches it to the embed (attachment://xp-chart.png). The SVG is generated in src/utils/xpChart.js and rasterized with sharp — already a dependency — lazy-required so tests and non-chart paths never load the binary. The SVG deliberately contains no <text> elements: node:20-slim ships no fontconfig fonts, so labels would render blank; the date range is conveyed in the embed footer instead.

Behavior:

• ≥ 2 days of history → chart attached, footer reads "XP over the last N days". Today's point is always synthesized from the live total so the chart ends at the true value even before today's award writes back.
• < 2 days (new users / pre-existing docs) → plain embed with footer "XP history builds over time — check back tomorrow". History starts accumulating from the next awardXP onward; xpByDay[today] correctly reflects the all-time total rather than only new XP.
• Chart render failure → caught, degrades to the stats embed without an image.

Tests

node --test — 550 tests pass, including 11 new in test/levelXpChart.test.js covering buildXPChartPoints (sorting, today upsert, malformed keys, 30-day cap), text-free SVG output, awardXP xpByDay writes/accumulation, and /level check attachment + footer behavior.