feat: add a demo clock for subscription renewals

[ovitrif]

Twin: synonymdev/bitkit-android#1341

This PR adds a demo clock so a recorded demo can show a subscription renewal without waiting a billing period.

Description

• Adds Dev Settings › PAYKIT › "Demo clock offset (days)" (Off, 1, 7, 30, 31, 62, 365) so subscription scheduling can be moved forward on a debug build
• Applies the offset to Paykit subscription scheduling only: proposal anchors and expiry, acceptance, due periods, renewal dates and due notifications, while one-time requests, invoices, payments and allowances keep real time
• Keeps the offset Off by default and ignores it outside debug builds, so release behaviour is unchanged

Out of Scope

• Device clock: unchanged; moving the simulator clock breaks the Pubky sessions, which is why the offset lives in the app
• Release builds: the row and the offset are unavailable there

Design

N/A — no design available.

Preview

Screenshots follow before review.

QA Notes

Journeys

• new second-billing-period.xml — with a 31-day offset the next period comes due as a Payment Request and the subscription detail lists two payments

Manual Tests

N/A

Automated Checks

• added DemoClockTests.swift — clamping, presets and the offset applied to subscription dates
• updated PaykitPaymentRequestServiceTests.swift — a demo clock offset makes the next billing period due