fix(theme): round short reset durations instead of truncating

[fix2it]

Summary

Improves duration_short formatting by replacing truncation with human-intuitive mathematical rounding, half-step (0.5) precision for small values, and seamless transitions between units.

Motivation & Problem

Previously, duration_short used integer truncation (floor behavior):

• A reset due in 2h 55m was displayed as 2h (giving the user the impression that almost an entire hour had vanished).
• A reset due in 4d 23h displayed as 4d.
• Below 1 hour, 59m 40s was indistinguishable from 59m.

Key Changes

1. Half-Step Precision (< 10):
   • For values $< 10$ hours or $< 10$ days, values are rounded to the nearest 0.5 step with a period separator (e.g., 1.5h, 2.5d, 9.5h).
   • Whole numbers within this range omit the decimal point (e.g., 3h, 6d).
2. Standard Rounding (>= 10):
   • For values $\ge 10$ hours or $\ge 10$ days, integer mathematical rounding is applied (16h, 10d).
3. Boundary Transitions & Rollover:
   • Durations $< 1$ hour switch strictly to minutes and are clamped with .min(59) so that the final seconds before 1 hour never show an awkward 60m before flipping to 1h.
   • Durations near 24 hours (e.g., 23h 45m) roll over cleanly to 1d rather than displaying 24h.

Verification & Tests

• Added comprehensive unit tests in src/theme_engine/tests.rs (duration_short_precision_scale_and_half_steps) verifying:
  • Sub-minute values (45s, "now").
  • Minute bounds (60s -> 1m, 3580s -> 59m).
  • Half-hour and whole-hour steps (1h, 1.5h, 2.5h, 9.5h, 10h, 18h, 85500s -> 1d).
  • Half-day and whole-day steps (1d, 1.5d, 2.5d, 6.5d, 10d).
• All 101 theme engine tests pass.