fix(test): use the intended mobile viewport in chat-sheet browser coverage

The chat-sheet browser runner creates an isMobile context with viewport402x874, but its fixture HTML has no viewport meta tag. A real Chromium probe at this configured size reports innerWidth980, visualViewport.width980 and scale0.4102. The suite therefore exercises a scaled desktop-width layout while labeling captures mobile. Its restore helper contains a keyboard fallback specifically for the widened mobile surface, increasing fixture complexity and bypassing one touch path.

Correct this fixture to use the device-width viewport, verify actual browser layout width, and remove the workaround only if the real touch restore path passes at the intended width. Preserve the compiled-style composer checks and retained desktop/mobile behaviors. This changes test infrastructure only; no product UI changes are proposed. Existing screenshots and probe are locally recorded; no full-device certification is claimed.