fix(ui): cover tiles keep the monogram for landscape images; quiet pill header nav

Landscape images (imported video frames, slide pages) would lose most of the
picture in a 3:4 crop, so the tile keeps its monogram; portrait and
near-square images (w/h <= 1.05) fade in over it, cropped from the centre.
The header nav follows the mockup's segmented-control style: muted text, no
underline, the current area raised in accent.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ui/src/index.ts

@@ -1,2 +1,2 @@
 export { AppShell } from "./AppShell";
-export { Pill, SegmentedToggle, ProgressBar, FloatingActionBar, CourseCover, monogram } from "./primitives";
+export { Pill, SegmentedToggle, ProgressBar, FloatingActionBar, CourseCover, coverFits, monogram } from "./primitives";