[Suggestion] 30-day & 12-month sparkline trends in /usage #8

Is your feature request related to a problem? Please describe.
The /usage dialog shows single-period numbers and a % change vs the previous period, but there is no way to see the overall trend — when you are busy, when you are quiet, how usage evolves across weeks.

Describe the solution you'd like
Add a "Trends" section to the /usage dialog:

• New g key toggles the trends section for the currently selected granularity:
  • 30-day sparkline when in month view (one ▁▂▃▄▅▆▇█-style bar per day)
  • 12-month sparkline when in month view (one bar per month over the last year)
  • Weekly/daily views show a fitting window (e.g. 12 weeks / 30 days)
  • "Busiest day of week" line (e.g. "Peak: Wednesdays")
• New pure helper makeSparkline(values: number[], height) in model-usage/helpers (adjacent to format.ts) + unit tests
• New db helper queryDailyTotals(startMs, endMs) in model-usage/db.ts — single GROUP BY over the day (no per-row JS work)
• Prefer existing hierarchy cache (days already built by buildHierarchy) before touching the DB

Describe alternatives you've considered

• Third-party chart libs — rejected, no dependencies; ASCII block glyphs fit the TUI perfectly
• Adding trends to the sidebar — rejected; sidebar stays compact

Additional context
The sparkline helper should be pure and reusable (candidate for opencode-wlib later).

Media
N/A