refactor(pixmap): route all panel icons through BlocksPixmap

[gmmcosta15]

Description

• Code refactor

• Convert every remaining handwritten panel and widget.

• Use source() at the 3 full-bleed sites: slider background, babystep graphic, probe helper graphic.

• Add stylesheet url(:/...) scanning to the resource guard.

• Net 116 lines removed across 39 files.

Motivation

Completes the stack: one object owns every pixmap, no handwritten panel holds a raw literal.

drawPixmap clips rather than stretches when the source rect exceeds the pixmap, so full-bleed art needs source(), not a 128x128 raster.

The guard could not match url(:/...), which has no quote next to the :. 17 such sites across 11 panels were invisible to it and only looked covered because the same keys appear quoted elsewhere.

Generated UI modules are out of scope; controlStackedWidget_ui.py is hand-edited and must not be regenerated.

Tests

make check green (pylint 8.27/10, unchanged); docstr-coverage 80.3%.

Screenshots

Pre-existing and unchanged from dev: the babystep/probe-helper graphic is a 346x276 asset in a 121px label with setScaledContents(False), so it is vertically centre-cropped.

Future work

• Convert the 79 raw literals in the generated UI modules.
• Reorganise the resource tree, rename the assets to one convention, add a manifest.
• Measure RSS on the RF50 against the 404400 kB baseline.