test(frontend): lock the style invariants a QA pass keeps re-finding

Two defects in this QA pass were pure CSS drift with no test surface at all:
100vw full-bleed sections panning the document by a scrollbar's width, and
controls sitting on three different corner radii. Assert both against the
sources, so the next person who hard-codes a radius or drops an overflow-x
guard fails CI instead of failing a screenshot review.

Tests now read files off disk, so they live in their own tsc project
(tsconfig.test.json, node types) excluded from the browser build.

frontend/src/features/landing/tests/style.test.ts

@@ -0,0 +1,33 @@
+import { readFileSync } from 'node:fs'
+import { expect, test } from 'vitest'
+
+/**
+ * Style invariants. These are what a visual QA pass keeps re-finding, because nothing
+ * else stops them from regressing silently: a full-bleed section turning into a
+ * horizontal scrollbar, and controls drifting onto a corner radius different from the
+ * button sitting next to them. Reads the sources, so it is a rule about the CSS we ship,
+ * not about one browser's computed output.
+ */
+// vitest runs from the package root, the same place `bun run build` does.
+const read = (rel: string) => readFileSync(rel, 'utf8')
+
+const indexCss = read('src/index.css')
+const landingCss = read('src/features/landing/styles/landing.css')
+const button = read('src/shared/ui/Button.tsx')
+const textField = read('src/shared/ui/text-field/TextField.tsx')
+
+test('full-bleed 100vw sections cannot produce a horizontal scrollbar', () => {
+  expect(landingCss).toMatch(/\.lv-bleed\s*{[^}]*width:\s*100vw/)
+  // Clipping body alone is not enough: <html> still pans by the width the 100vw
+  // sections overshoot the client width by.
+  expect(indexCss).toMatch(/html\s*{[^}]*overflow-x:\s*clip/)
+  expect(indexCss).toMatch(/body\s*{[^}]*overflow-x:\s*clip/)
+})
+
+test('every control shares one corner radius', () => {
+  expect(indexCss).toMatch(/--radius-ctl:\s*10px/)
+  expect(button).toContain('rounded-ctl')
+  expect(textField).toContain('rounded-ctl')
+  // A hard-coded radius here is how the CTAs ended up at 9.6px while buttons sat at 6px.
+  expect(landingCss).not.toMatch(/\.lv-cta[^{]*\{[^}]*border-radius:\s*(?!var\(--radius-ctl)\d/)
+})