Theme: expose tertiary on the singleton (was undefined in dynamic mode) (#2375)

The dynamic-mode currentThemeData object at Theme.qml:447-468 omitted
tertiary entirely, even though matugen generates it and writes it to
the matugen colors JSON. As a result, Theme.tertiary returned undefined
in dynamic mode, and any QML binding through .r/.g/.b/.a defaulted to
white — visible as desaturated near-white highlights in the bar's
ChromeShader (which mixes Theme.tertiary at highlight peaks).

Two-line fix:
- Add tertiary to the dynamic-mode currentThemeData via getMatugenColor
- Add property color tertiary on the Theme singleton, falling back to
  secondary if tertiary is absent (covers the stock-theme path where
  tertiary is added separately via buildMatugenColorsFromTheme:1811)

Fallback hex #efb8c8 is the M3 baseline tertiary (light pink) for dark
mode, in case matugen output is missing.

Theme.tertiary was already referenced at LockScreenContent.qml:737 so
the property name is the right one.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

quickshell/Common/Theme.qml

@@ -450,6 +450,7 @@ Singleton {
                 "primaryText": getMatugenColor("on_primary", "#ffffff"),
                 "primaryContainer": getMatugenColor("primary_container", "#1976d2"),
                 "secondary": getMatugenColor("secondary", "#8ab4f8"),
+                "tertiary": getMatugenColor("tertiary", "#efb8c8"),
                 "surface": getMatugenColor("surface", "#1a1c1e"),
                 "surfaceText": getMatugenColor("on_background", "#e3e8ef"),
                 "surfaceVariant": getMatugenColor("surface_variant", "#44464f"),
@@ -522,6 +523,7 @@ Singleton {
     property color primaryText: currentThemeData.primaryText
     property color primaryContainer: currentThemeData.primaryContainer
     property color secondary: currentThemeData.secondary
+    property color tertiary: currentThemeData.tertiary || currentThemeData.secondary
     property color surface: currentThemeData.surface
     property color surfaceText: currentThemeData.surfaceText
     property color surfaceVariant: currentThemeData.surfaceVariant