fix(website): adaptive FPV FOV + landscape fullscreen for the Parthenon example

[apresmoi]

Follow-up to #31 fixing two mobile issues on /examples/parthenon.

1. Portrait FPV FOV was too "close"

The FPV camera used a fixed CSS-perspective focal length (perspective = 1000). Horizontal FOV ≈ atan((width/2) / P), so on a ~414px-wide portrait phone that gave ~22° (telephoto/claustrophobic) vs ~56° on desktop.

fpvPerspective() now scales P with the stage width (anchored so a desktop-width stage keeps the original value), holding a constant ~56° horizontal FOV across phone/tablet/desktop, and re-adapts on rotation/resize.

	host width	perspective	horizontal FOV
Desktop	1059px	999	55.8°
Portrait (before)	414px	1000	~22°
Portrait (now)	414px	391	55.8°

2. Rotating to landscape cramped / covered the screen

The compact mobile layout (sidebar → tab bar, verbose header hidden) was gated only on max-width: 50rem, which a landscape phone (896px wide) fails — so it fell back to the desktop layout, squishing the temple into a ~154px sliver behind the left rail + header.

• Added a (max-height: 34rem) trigger to the compact-layout breakpoints (ExamplesSidebar.astro + the page dock) so short/landscape viewports get the compact layout. Benefits every example page in landscape.
• On landscape phones the page now goes fullscreen: the site nav (.header), the examples sidebar, and the page header are hidden so the matrix fills the whole screen (landscape stage height 154px → full viewport). Rotating back to portrait restores all chrome + navigation.

Notes

• Website-only; no library/API changes.
• Tested on-device across portrait/landscape.