fix(qt): glyph colours follow the theme, on light palettes and on accent fills

Two defects with one cause: a tint name that means a frozen hex.

"primary" is the legacy alias of literal #ffffff, not Theme.text-primary,
so every control passing it drew a white glyph. On the eleven light
themes that is white-on-light -- the header back/forward chevrons, the
sidebar toggle and the carousel arrows the owner marked. They now pass
the theme-following names, which icon_tint_qt bakes at runtime.

The stale `hslLightness > 0.5 ? "primary" : "black"` workarounds that
predate the runtime tinter collapse into those names, and the qrc floor
underneath them becomes polarity-aware: dir_for() returns "" whenever
the cache dir is unavailable, a bake fails or the theme document has not
been published, and that path used to resolve textPrimary straight back
to the white bake -- reproducing the bug it was fixing, app-wide, in the
degraded state.

On accent fills the fix is a deviation from Slint, approved by the owner
after measurement. A fixed white glyph on Theme.accent falls under 2.6:1
on 16 of the 35 shipped palettes, including high-contrast at 1.70:1 and
wcag-dark at 1.82:1 -- the two themes whose reason to exist is contrast.
Slint hardcodes #ffffff there (CircleAction.slint:72 and three others),
so the bug is upstream. accentText alone is not the answer either: it is
picked for the theme's text ramp and gives 2.56:1 on Rose Pine Dawn,
worse than the black it replaces. So QbzTheme picks the best-contrast
tint by real sRGB luminance against the live accent, and the labels
beside those glyphs use its colour twin -- BrowseGenreButton and
LibraryView were drawing a white glyph next to an accentText label
inside the same pill.

Rounds out the two parity items the same files owe: QbzNavButton gains
the ambient arm from Carousel.slint (wired at the six carousel sites,
not left dead), and QbzCircleAction rounds its glyph to whole pixels the
way CircleAction.slint does.

cc

crates/qbz-qt/qml/controls/CardMenu.qml

@@ -73,7 +73,10 @@ QbzContextMenu {
                     width: 15
                     height: 15
                     anchors.verticalCenter: parent.verticalCenter
-                    tintName: row.hot ? "primary" : "secondary"
+                    // Host is `surfaceHover`/transparent over a theme surface,
+                    // so the glyph tracks the sibling label (textPrimary/
+                    // textSecondary below) rather than a fixed white.
+                    tintName: row.hot ? "textPrimary" : "secondary"
                 }
                 Text {
                     height: parent.height