Removes the IBM Plex Mono Google Fonts @import, replaces the --font CSS
variable with a cross-platform system sans-serif stack, and adds a
--font-mono variable for the share-URL readonly input so copy/share text
stays monospace. Also adds line-height: 1.5 to body to compensate for
the system fonts' tighter default leading.
No font asset files exist in /public; layout.ts has no font <link> tags
to remove. Acceptance check: build + 112 tests pass; new
tests/integration/style.test.ts asserts no @import / IBM Plex / external
font URL remains and that the system stack is wired up.
