Fix/device time layout and screenshots

[bschlagheck]

Summary

• improve the device time layout in the top bar
• show timezone separately as a compact badge
• align mock timezone output with real device behavior (CET / CEST)
• update the README mobile screenshots

Validation

• rebuilt embedded web assets
• ran make build successfully

[Friedjof]

I reviewed this PR and did not find a blocking issue. Checks are green and the changes look safe to merge.

One small non-blocking note: the mock now derives CET / CEST from the browser's local timezone offset. That is fine for the real device behavior this UI targets, but contributors outside the CET/CEST timezone may see a less representative mock label. Not a blocker from my side.