feat(usage): time-grid usage bars with current-segment highlight and over-pace warning

[jonasnobile]

Summary

Makes the Claude and Codex usage popovers show whether you're in line with your rate limits — not just how full the bar is. Three layered cues:

1. Per-hour / per-day grid. Each usage bar is split into time segments — per-hour for the 5h session window, per-day for the 7d windows. Codex derives the segment count from the window's actual duration (capped so thin bars don't get crowded; sub-hour windows get no grid). Dividers use the border colour so they read over both the filled and empty parts of the track.

2. Current-segment highlight. The segment you're currently in (today / this hour) gets a translucent band derived from text_primary, so it lightens on dark themes and darkens on light ones.

3. Over-pace warning. Previously the bar only changed at 80%+ absolute. Now:

   • The fill up to the pace marker is the on-budget portion (coloured by nearness to the hard cap); usage beyond the marker is drawn as overage in warning/critical, so being over budget for this point in the period is visible directly on the bar.
   • The percentage text reflects the worse of absolute-vs-cap and pace.
   • A "Slow down to last the period" (critical) / "Ahead of pace" (warning) line appears when usage is running ahead of schedule.

Notes

• Pace uses even-burn pacing (budget should be spent linearly across the window). A user who deliberately front-loads work early in a 7d window will see "Ahead of pace" even if they'll be fine — consistent with how the bar fill already behaved, just now surfaced.
• The grid is a pace reference over the current utilisation snapshot, not historical per-bucket consumption (the usage APIs return a single cumulative figure per window).

Test plan

• cargo build — both crates compile
• cargo clippy — clean
• cargo test — all pass (added segments_for_window unit test in Codex)
• Visual check of both popovers in the running app (overage cap, highlight band, message line)

🤖 Generated with Claude Code

[jonasnobile]