Refine landing-page shadows, lamp glass, and flexible postcard

[petekp]

The landing page now has a refractive, glowing lamp, shadows that stretch and soften with distance, and a postcard that bends while its controls stay live. This addresses rigid motion, fuzzy edges, and shadows that overlapped the lifted card or outlined the heading.

Built on #84. The diff contains only the landing-page refinements and their browser probes.

Verification	Result
Tests	126 files, 1,589 tests passed
Probe source edits	Missing and ambiguous markers fail; spacing regression checked
Typecheck, lint, package build, website build	Passed
GitHub CI	All three jobs passed for e10d0c7: checks, browser-gates, knobs-resize
Chrome: lamp refraction and zoom, postcard edges and shadows, scrolling, typing, mobile, native fallback	Passed
Six postcard lift-and-return cycles	No pixel change at the 12 handoff boundaries
Page shadow softness with the lamp moved farther across the page	Edge transition widened from 9.1 to 22.2 CSS pixels

Review the geometry shared by paper rendering, hit testing, and shadows, plus the native-resolution lighting pass in homePaperLighting.ts. Those paths must stay aligned through handoffs, scrolling, and zoom. Page refraction requires HTML-in-canvas support; the fallback retains the lamp's glass reflections and glow.