feat: Today's Pulse — same-elapsed usage comparison in the dashboard rail

[pitimon]

Summary

• Replaces the redundant StatsPanel rolling-tiles + top-models (which duplicated the Hero period tabs and the Provider Breakdown) with a lean Today's Pulse card: Tokens / Cost / $-per-MTok, each vs an apples-to-apples same-elapsed baseline (partial "today" compared only up to the current clock hour — no "mornings always look low").
• Period-aware: Day → 2 deltas (vs yesterday · vs 7-day trailing average); 7d/30d → 1 delta (vs previous window); Total → hidden.
• Semantic colour, not just direction: usage ▲ = amber (attention), $/MTok ▲ = red (pricier/worse), $/MTok ▼ = green (cheaper/better).
• Server (2 lines): aggregateHourlyByDay now sums total_cost_usd per bucket (mirrors aggregateByDay) so cost & $/MTok can be computed same-elapsed. Reuses the existing computeRowCost.
• Fetch is minimal: hourly only for the partial/boundary days, daily aggregates for completed days → ~2–8 requests, no 60-request cliff.
• Pulse math is a pure, unit-tested module (pulse-stats.ts); StatsPanel decluttered to the identity block.

Test plan

• pulse-stats unit tests (17) — truncation, ratio-of-sums $/MTok, div-by-zero guards, period-aware deltas
• StatsPanel tests rewritten for the decluttered panel (3)
• Full dashboard vitest: 272 pass
• ci:local green — root 775 tests, copy, ui-hardcode, guardrails 4/4
• Live E2E (local serve, real data): Day = 2 deltas @ same clock time with correct amber/green/red colours; 7d = single "vs prev 7d" delta (hybrid daily+hourly); Total = pulse hidden
• Post-merge: version bump → publish → scripts/release.sh → Playwright verify on :17680