feat(usage): anchor day/hour grid to each user's actual reset time

[jonasnobile]

Summary

Follow-up to #137. The usage-bar grid was a plain even split (i / segments), which only coincidentally lined up with day/hour boundaries for clean windows — it wasn't actually derived from the reset. It drifted for any other reset time, for non-integer windows (Codex 30d → 14 even blocks, 1d → 12 even blocks), and across DST.

Now boundaries are computed from the real reset timestamp using jiff calendar arithmetic, so blocks land on the user's own reset time-of-day. A Sun 12:00 weekly reset yields noon-to-noon day blocks; a user whose reset is Wed 03:00 gets blocks anchored there. The current-segment highlight uses the same boundaries, so it always marks the real "today" / "this hour" block.

Changes

• Claude: thread the raw ISO reset + window length through TierUsage. Session → hour granularity; the 7-day tiers → day granularity. New reset_aligned_segments() returns the divider fractions + current-block range.
• Codex: add jiff; derive boundaries from the epoch reset_at + window_seconds, choosing hour/day granularity per window (segment_unit_for_window); sub-hour windows get no grid.
• Long windows step by a whole multiple of the unit (e.g. every 2 days) so a thin bar stays legible while staying reset-anchored.
• Replaced the old segments_for_window test with segment_unit_for_window + reset_aligned_segments tests (guards + weekly-window invariants).

Notes

• Boundaries use wall-clock calendar steps (jiff), so a block stays "noon to noon" across a DST change even though that block is 23h/25h of real time. The pace marker still uses the fixed-seconds elapsed %, so across a DST transition the marker and a boundary can sit up to ~1h apart for that one block — acceptable and rare.
• For the common 7-day / 5-hour windows in a DST-free week this is visually identical to the previous even split — the change is correctness for other reset times, odd window lengths, and DST.

Test plan

• cargo build (pinned 1.95.0 toolchain) — both crates
• cargo clippy — clean
• cargo test — 28 pass
• Visual check in the running app that the weekly grid lines sit on local noon and "today" is highlighted

🤖 Generated with Claude Code