fix(landscape): clip grid cells and stop egui's interact_size floor overflowing compact panels

In compact landscape profiles (e.g. landscape-qhd-top, sc~=0.74), two related
bugs caused panel content to overflow its allocated space instead of being
contained: egui's default Style::interact_size.y (18px, unscaled) sets a
floor on row height regardless of our own sc-scaled constants, and
UiBuilder::max_rect is a layout hint, not a clip boundary, so content that
overflowed anyway wasn't contained.

- dashboard.rs::render_landscape_grid now explicitly clips each grid cell to
  its cell_rect.
- cpu.rs's per-core scroll viewport gets an explicit clip rect and overrides
  interact_size.y so exactly 2 rows (4 cores) render, matching the intended
  scrollable-viewport design instead of bleeding into the sparkline below.
- disk.rs's drive-list ScrollArea overrides interact_size.y so drive rows
  render at their intended compact height instead of the last drive's model
  label getting cut off.

Verified visually in landscape-qhd-top (Desktop Wallpaper mode) and
portrait-xl (no regression).

Closes #171
Closes #172