[tech-debt] Shared layout constants: HEADER_HEIGHT (64) + LG_BREAKPOINT (1024)

[ignromanov]

From /simplify review of PR #274 (altitude finding).

64 (header height) is spelled 3 independent ways: (app)/layout.tsx calc comment + h-[calc(100dvh-104px)], top-[4rem] on the sticky tab bar, and CreateWorkspace.tsx scroll-offset el.offsetTop - 64 - tabBarH. 1024 (lg breakpoint) is inlined as window.matchMedia('(min-width: 1024px)') duplicating Tailwind lg.

Scope: introduce HEADER_HEIGHT_PX + LG_BREAKPOINT in shared/config; derive the calc, scroll-offset, and JS guard from them. Silent drift risk if header resizes.

Out of scope of PR #274 (touches (app)/layout.tsx + tokens).