Align fixed-quote journeys with both payment flows #1610

The fixed-quote payment journeys differ between Android master (after #1581 merged) and iOS #1014. The apps intentionally expose LNURL/Savings selection on different screens, but the Android journey and README do not document that distinction or include the explicit two-rail LNURL case.

Reported in iOS #1014 review. This is a non-blocking QA/documentation follow-up; no new runtime payment defect was found in that review.

Update journeys/payment-requests/fixed-quoted-confirmation.xml and its README:

• Specify Savings in the first fixed USD request case and Savings plus BOLT11 Spending in the direct-confirmation rail-switching case, matching the iOS wording.
• Cover a fixed USD request offering LNURL Spending and Savings with different quoted amounts. On Android, Confirm must offer both wallets, show the chosen rail's exact quote and fees, and still require explicit payment authorization. Cover LNURL-only direct confirmation too. Stop before paying.
• Record the same platform distinction on both sides: iOS retains Amount for LNURL plus Savings because its LNURL confirmation has no funding selector; Android offers the choice on Confirm. Keep shared instructions aligned and express platform differences explicitly.
• Check the README's existing fixed-price paragraph against the merged routing behavior; it still describes an Amount → Continue step and says differing rail prices are unsupported.

Evidence: the current Android master journey has the older description and two broader “Bitcoin” cases, and no final LNURL/Savings action. iOS #1014 at 7dbb5e05d70bc203cf58bb295bf23ba224440ea1 contains those additions. Android #1581 is merged at 3cdc79c5826c143774c3aa039a065768874ee1d0; keep the follow-up separate from that merged PR.

Validation: parse the changed XML, compare the intended shared prose and platform notes, and drive the affected regtest confirmation cases on both platforms without sending payment. No new device run is claimed by this report.