test(mobile): read PHONE_MAX as an exclusive bound everywhere, drop the stale 430px baselines

Follow-up to #390. PHONE_MAX had become 599, an inclusive bound, while
three of its four consumers still read it as exclusive (width < PHONE_MAX
for phone); the one site that switched to <= disagreed with
getDeviceType(). It is 600 again with < at every site. The breakpoint
table in docs/mobile-testing-report.md says 600, and the three 430px
visual baselines are removed: they depict the tablet tier now, and the
visual suite recreates a missing baseline on its next run on the machine
that owns them. device-matrix.test.ts is also run through Prettier, which
the commit hook demanded and the format gate (src/ only) never did.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

test/mobile/helpers/constants.ts

@@ -59,7 +59,7 @@ export const SELECTORS = {
 
 // Device breakpoints (match app.js MobileDetection)
 export const BREAKPOINTS = {
-  PHONE_MAX: 599,
+  PHONE_MAX: 600,
   TABLET_MAX: 768,
 } as const;