render: match Chrome font metrics with Liberation Sans/Serif/Mono

Chrome's default sans-serif on this class of host is Liberation Sans, not
DejaVu (measured: sans-serif == Arial == Helvetica == Liberation width,
DejaVu ~12% wider). Forcing DejaVu drifted all text wider and shifted
every line. Bundle Liberation Sans/Serif/Mono (DejaVu kept as glyph
fallback), default to Liberation Sans, and resolve font-family to the
right generic (monospace/code -> Mono, serif -> Serif, else -> Sans).
A controlled text page drops from ~15-20% to 3.5% pixel mismatch and
every line lands within 1px of Chrome.

crates/obscura-render/src/dom.rs

@@ -309,6 +309,7 @@ pub fn layout_dom_with_images(
             color: Option<[u8; 4]>,
             font_size: Option<f32>,
             font_weight: Option<String>,
+            font_family: Option<String>,
             visibility_hidden: bool,
             opacity_product: f32,
             list_style: crate::ListStyle,
@@ -328,6 +329,7 @@ pub fn layout_dom_with_images(
                     color: None,
                     font_size: None,
                     font_weight: None,
+                    font_family: None,
                     visibility_hidden: false,
                     opacity_product: 1.0,
                     // CSS initial value of list-style-type.
@@ -400,6 +402,7 @@ pub fn layout_dom_with_images(
                     }
                 }
                 match &style.font_weight { Some(w) => inh.font_weight = Some(w.clone()), None => style.font_weight = inh.font_weight.clone() }
+                match &style.font_family { Some(f) => inh.font_family = Some(f.clone()), None => style.font_family = inh.font_family.clone() }
                 inh.visibility_hidden = style.visibility_hidden.unwrap_or(inh.visibility_hidden);
                 inh.opacity_product *= style.opacity.unwrap_or(1.0);
                 style.effectively_invisible = inh.visibility_hidden || inh.opacity_product < 0.02;