refactor(style): drop custom webfont, use system fonts

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.

public/style.css

@@ -1,5 +1,3 @@
-@import url('https://fonts.googleapis.com/css2?family=IBM+Plex+Mono:ital,wght@0,400;0,500;0,600;1,400&display=swap');
-
 /* ── Reset ──────────────────────────────────────────────── */
 *, *::before, *::after {
   box-sizing: border-box;
@@ -18,7 +16,8 @@
   --gray-600:#555;
   --red:     #c00;
 
-  --font: 'IBM Plex Mono', 'Courier New', monospace;
+  --font: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
+  --font-mono: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
   --border: 1px solid var(--black);
   --radius: 0;
 }
@@ -28,6 +27,7 @@ html { font-size: 14px; }
 
 body {
   font-family: var(--font);
+  line-height: 1.5;
   background: var(--white);
   color: var(--black);
   min-height: 100vh;
@@ -265,6 +265,7 @@ a.btn:hover {
 
 .share-box input[readonly] {
   flex: 1;
+  font-family: var(--font-mono);
   background: var(--gray-100);
   color: var(--gray-600);
   border-right: none;