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/views/local/SelectCheck.qml

@@ -33,7 +33,19 @@ Rectangle {
         width: Math.round(root.diameter * 0.62)
         height: Math.round(root.diameter * 0.62)
         anchors.centerIn: parent
-        tintName: "primary"
+        // DELIBERATE DIVERGENCE from primitives/SelectionCheckbox.slint:37,
+        // which hardcodes `tint: #ffffff` on this same accent fill. That
+        // white is under 2.6:1 on 16 of the 35 palettes, worst on
+        // high-contrast at 1.70:1, then ikari 1.74 and wcag-dark 1.82 —
+        // i.e. the two ACCESSIBILITY themes bracket the top of the list.
+        // (This site used to claim ikari 1.74 was "the worst case in the
+        // app"; it is second. The 8px glyph here does make it the smallest
+        // thing painted at those ratios, which is a different argument and
+        // the one worth keeping.) Slint's own QbzCheckbox.slint:24 uses
+        // accent-text for the equivalent glyph, so upstream contradicts
+        // ITSELF here; the selector settles it by measurement instead of by
+        // picking a side. See theme/QbzTheme.qml, "ON AN ACCENT FILL".
+        tintName: theme.accentGlyphTint
     }
     MouseArea {
         id: checkArea