Bundle Geist as the body sans

The design's `--ely-font-sans` is Geist; we were registering only
Newsreader. Body text everywhere fell through to GPUI's
`.SystemUIFont` (SF Pro on macOS) — too humanist for the geometric,
calm tech aesthetic the design wants.

Bundle Geist-Regular (SIL OFL 1.1, 126 KB), register it alongside
Newsreader, and set the root window's `.font_family(SANS_FAMILY)`.
SERIF_FAMILY callsites continue to override per element so headlines
keep using Newsreader.

Only Regular is bundled — GPUI synthesizes weights from the metrics,
so 500/600 still read correctly. Bold/italic file additions are a
later polish.

crates/ely_app/src/shell/chrome/mod.rs

@@ -30,5 +30,5 @@ pub(crate) use split_pane::{
     render_split_pane_header,
 };
 pub(crate) use topbar::render_topbar;
-pub(crate) use typography::{SERIF_FAMILY, register_serif_fonts};
+pub(crate) use typography::{SANS_FAMILY, SERIF_FAMILY, register_serif_fonts};
 pub(crate) use wallpaper::render_wallpaper;