Fix segment count to 7 for weekly (and optionally session) usage bar

[Lexx-Agency-dev]

Problem

Both the 5h and 7d usage bars currently render with the same fixed
segment count (SEGMENT_COUNT = 10 in src/window.rs, used via
row_bar_segment_count when only one model is active). For the 7-day
window this doesn't map onto 7 days, making the bar less intuitive to
read at a glance.

Change

Changed SEGMENT_COUNT from 10 to 7, so each segment in the bar
corresponds to one day for the 7d window (and, since both rows share
this constant, also gives the 5h bar 7 segments for visual consistency).

- const SEGMENT_COUNT: i32 = 10;
+ const SEGMENT_COUNT: i32 = 7;

Scope / impact

• Only affects the active_models == 1 case (single model shown).
  The 2-model (5 segments) and 3-model (4 segments) layouts in
  row_bar_segment_count are unchanged.
• No layout/width bugs: bar width, text position and widget width are
  all computed from segment_count, so they scale automatically.
• No new dependencies, no unrelated files touched.

Testing

Built locally with cargo build --release and confirmed the taskbar
widget renders both bars with 7 segments, filling proportionally to
the reported usage percentage.

Screenshots

Before: [10 segments]
After: [7 segments]