feat(sensors): DPI-aware column density, bordered sections

[perlowja]

Makes the sensors detail popover responsive using logical monitor pixels only and keeps the balanced multi-column layout.

The sensors-resource-section rule now lives in the plugin own compact_rows_provider, so the plugin no longer depends on libsingularity for plugin-specific styling.

Verified by building sensors/libsensors.so with Meson and Ninja in a Debian trixie container against the current libsingularity fallback. The 1280 logical-pixel acceptance calculation produces one 340-pixel column at either scale 1 or scale 2.

AI assistance: disclosed

[perlowja]

@mirkobrombin bumping this one - restores the bordered-section styling and DPI-aware column density that got dropped when sensors moved from panel.vala into this plugin. Depends on libsingularity#15 for the CSS class.

[mirkobrombin]

The scale factor logic is backwards: popovers are laid out in logical pixels, so multiplying by scale gives HiDPI screens more columns on less space (1280 logical @2x gets 3 columns, a ~1056px popover). Keep the logical-only calculation. Also move .sensors-resource-section into the plugin's own CSS provider (compact_rows_provider), libsingularity#15 is closed.

[perlowja]

@mirkobrombin both points from your review are fixed in 05d6471 (rebased on main d255d47):

• the column count uses the logical popover width only; the scale-factor term is gone
• the .sensors-resource-section style now lives in the plugin's compact_rows_provider, so no libsingularity change is needed

Built on main with shell#30 and desktop#262 on forky arm64: ninja 379/379 targets, meson test 23/23 pass. Please take another look.