fix: clear render and text caches on theme change in PassPolarPlotControl

[Copilot]

PassPolarPlotControl cached FormattedText and render resources (SolidColorBrush, Pen) but never invalidated those caches on theme change, causing N/E/S/W cardinal labels and plot geometry to render with stale palette colours after switching themes.

Changes

• PassPolarPlotControl — overrides OnAttachedToVisualTree/OnDetachedFromVisualTree to subscribe/unsubscribe from ActualThemeVariantChanged, clearing _renderCache and _textCache on each theme switch. Mirrors the existing pattern in SkyPlotControl and WorldMapControl.

• FormattedTextCache — the Get(name, fontSize, Color foreground) overload used only (name, fontSize) as the cache key, so a colour change for the same text+size silently returned a stale entry. Key expanded to (name, fontSize, foreground):

// Before — foreground colour not part of key
var key = (name, fontSize);

// After — colour change produces a distinct cache entry
var key = (name, fontSize, foreground);