frontend: LN screens replace back button to chevron

[shonsirsha]

To maintain the UI's consistency, as what we did in #4197 we are replacing the back button in various LN-related pages to Chevron back on the header (mobile).

Eight routes were updated, covering 21 views/stages and replacing 20 body Back/Cancel controls on mobile.

Route	Impacted views/stages
/lightning/send	Manual payment entry; BOLT11 review; LNURL Pay review; Bitcoin address/BIP21 review
/lightning/receive	Lightning-address view; create-invoice form; generated-invoice view
/lightning/topup	Top-up form; no-Bitcoin-account fallback; no-funded-Bitcoin-account fallback
/lightning/activate	Intro; information/consent; embedded disclaimer; connect-device view
/lightning/deactivate	Intro/confirmation view
/lightning/set-lnurl-address	Load-error view; address form
/lightning/claim-top-up	Overview; claim/refund confirmation
/lightning/close-withdraw-funds	No-Bitcoin-account fallback; close-and-withdraw confirmation

Behavior:

• Mobile (≤768px): header chevron.
• Desktop (>768px): body Back/Cancel button remains.
• "Busy" and "terminal" stages hide the chevron.

1. Busy: activating/deactivating, creating an invoice, saving an LN address, submitting a claim/refund, or closing the wallet.

2. Terminal: success screens after Send, Receive, Top-up, activation, deactivation, claim/refund, etc.

Not converted:

• /lightning -> top-level page, no Back control.
• /lightning/disclaimer -> already used a header chevron.
• /settings/lightning-settings -> already used a header chevron.
• Send QR scanner -> retains its fullscreen X.
• Claim Top-up failure Cancel -> intentionally remains in the body.
• Close & Withdraw failure Cancel -> intentionally remains in the body.