site.yaml wordmark_height; raster logos keep their colors

Custom wordmark sizing per site (0.5-6 rem, clamped at render), and
the light-theme invert now applies only to .svg wordmarks - the
white-stroke house style - so a client's raster logo is never
recolored.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y42TyF8Zu7NGRR2893vNcZ

src/content.rs

@@ -513,6 +513,11 @@ pub struct SiteConfig {
     /// `None` falls back to `/wordmark.svg`.
     #[serde(default)]
     pub wordmark: Option<String>,
+    /// Wordmark display height in rem (0.5–6.0). `None` keeps the
+    /// stylesheet's default. Raster logos look best at or below their
+    /// intrinsic pixel height.
+    #[serde(default)]
+    pub wordmark_height: Option<f32>,
     #[serde(default)]
     pub hero: HeroConfig,
 }