Reserve the themed scrollbar gutter on the Popup and Notifications pages,
like Panel already does, so the viewport width no longer oscillates by 19
pixels while the content settles and the provider roster arrives. Bound the
Popup overview section to the same 24-grid width as its provider-order
sibling.

The new frame-based geometry test failed before the fix ('viewport padding
changed from 19 to 0') and passes after it, in both layout directions.

Validation: make check passed (920 Qt bulk tests, desktop-style geometry
runs, 77 Python tests, hardening, AppStream metadata). No CLI contracts,
configuration keys, translations, or TODO parity entries change.