feat(wallet): invoice-first lightning send — amount locks from the pasted invoice

A pasted fixed-amount invoice auto-fills and locks the amount ('set by
invoice' — nothing to type, just review and send); zero-amount invoices get
an explicit 'enter how many sats' hint. Plus a clipboard Paste button on the
destination field (hidden where clipboard read can't work).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

neode-ui/src/components/SendBitcoinModal.vue

@@ -67,6 +67,7 @@
         <div class="mb-3">
           <div class="flex items-center justify-between mb-1">
             <label class="text-white/60 text-sm">{{ t('sendBitcoin.amountSats') }}</label>
+            <span v-if="pastedInvoiceAmount !== null" class="text-[11px] px-2 py-0.5 rounded-full bg-white/10 text-white/50">set by invoice</span>
             <button
               v-if="sendMethod === 'onchain'"
               @click="toggleSendAll"
@@ -82,19 +83,34 @@
             v-model.number="amount"
             type="number"
             min="1"
-            :placeholder="sendAll ? '' : '1000'"
+            :placeholder="sendAll ? '' : pastedInvoiceAmount !== null ? '' : '1000'"
-            :disabled="sendAll"
+            :disabled="sendAll || pastedInvoiceAmount !== null"
             class="w-full input-glass disabled:opacity-50"
           />
           <p v-if="sendAll" class="text-xs text-white/50 mt-1">
             Sweeps your entire on-chain balance{{ onchainBalance !== null ? ` (~${onchainBalance.toLocaleString()} sats)` : '' }} minus network fees.
           </p>
+          <p v-else-if="pastedInvoiceAmount !== null" class="text-xs text-white/50 mt-1">
+            This invoice fixes the amount — nothing to type, just review and send.
+          </p>
+          <p v-else-if="effectiveMethod === 'lightning' && dest.trim()" class="text-xs text-white/50 mt-1">
+            Zero-amount invoice — enter how many sats to pay.
+          </p>
         </div>
 
         <div v-if="effectiveMethod !== 'ecash'" class="mb-3">
-          <label class="text-white/60 text-sm block mb-1">
+          <div class="flex items-center justify-between mb-1">
-            {{ effectiveMethod === 'lightning' ? t('sendBitcoin.lightningInvoice') : effectiveMethod === 'ark' ? 'Ark address, invoice or lightning address' : t('sendBitcoin.bitcoinAddress') }}
+            <label class="text-white/60 text-sm">
-          </label>
+              {{ effectiveMethod === 'lightning' ? t('sendBitcoin.lightningInvoice') : effectiveMethod === 'ark' ? 'Ark address, invoice or lightning address' : t('sendBitcoin.bitcoinAddress') }}
+            </label>
+            <button
+              v-if="canReadClipboard"
+              @click="pasteFromClipboard"
+              class="text-xs px-2 py-0.5 rounded border bg-white/5 border-white/15 text-white/60 hover:text-white/90 transition-colors"
+            >
+              {{ effectiveMethod === 'lightning' ? 'Paste invoice' : 'Paste' }}
+            </button>
+          </div>
           <textarea v-model="dest" rows="2" :placeholder="effectiveMethod === 'lightning' ? 'lnbc...' : effectiveMethod === 'ark' ? 'tark1… / lnbc… / user@lnaddress' : 'bc1...'" class="w-full input-glass font-mono"></textarea>
         </div>
 
@@ -177,6 +193,35 @@ function toggleSendAll() {
 // Leaving the on-chain tab disarms the sweep so it can never apply elsewhere
 watch(sendMethod, (m) => { if (m !== 'onchain') sendAll.value = false })
 
+// Invoice-first lightning UX: a pasted invoice that fixes its amount locks
+// the amount field (auto-filled, "set by invoice"); zero-amount invoices
+// leave it editable. Clearing/leaving lightning unlocks again.
+const pastedInvoiceAmount = computed<number | null>(() => {
+  if (effectiveMethod.value !== 'lightning') return null
+  const d = dest.value.trim()
+  if (!d) return null
+  return parseBolt11AmountSats(d.toLowerCase().startsWith('lightning:') ? d.slice(10) : d)
+})
+watch(pastedInvoiceAmount, (fixed, prev) => {
+  if (fixed !== null) amount.value = fixed
+  // Swapping a fixed-amount invoice for a zero-amount one: don't silently
+  // keep the previous invoice's sats — make the user type the new amount.
+  else if (prev !== null) amount.value = 0
+})
+
+// Clipboard read needs a secure context (or the companion bridge); hide the
+// button where it can't work — the textarea still accepts a manual paste.
+const canReadClipboard = typeof navigator !== 'undefined' && !!navigator.clipboard?.readText
+
+async function pasteFromClipboard() {
+  try {
+    const text = (await navigator.clipboard.readText()).trim()
+    if (text) dest.value = text
+  } catch {
+    // Permission denied — user can long-press/Ctrl+V into the field instead.
+  }
+}
+
 const effectiveMethod = computed(() => {
   if (sendMethod.value !== 'auto') return sendMethod.value
   const amt = amount.value || 0