support sub-sat fee rates in CAT-21 mint (port of PR #14)

Bitcoin Core now accepts sub-sat fee rates (e.g. 0.5 sat/vB).
Three changes to the cat21-mint component:

1. Fix BigInt crash: BigInt(vsize * feeRate) throws when result
   is fractional. Now uses Math.ceil() to round up to nearest sat.
2. Remove fullNumberValidator: users can now enter decimal fee rates.
3. Fix fee check toggle: was hardcoded to floor=1 (broken when
   recommended fee < 1). Now toggles between recommended and 0.

Our Bitcoin node (v30.2.0) already relays sub-sat txs (default
minrelaytxfee=1 sat/kvB = 0.001 sat/vB). No node config change needed.

Author: hans-crypto

frontend/src/app/components/_ordpool/cat21-mint/cat21-mint.component.html

@@ -62,8 +62,7 @@ <h2 class="mt-2 mb-4">Mint a CAT-21 Ordinal</h2>
 
               <div class="feedback-error p-1" *ngIf="cfeeRate.invalid">
                 <div *ngIf="cfeeRate.hasError('required')">Please enter a value!</div>
-                <div *ngIf="cfeeRate.hasError('min')">Enter at least a value of {{ minRequiredFee }} sat/vB! If you are really sure that you want a fee rate that is lower than the recommended rate, switch to Expert mode and deactivate the check.</div>
-                <div *ngIf="cfeeRate.hasError('notFullNumber')">Enter whole numbers only!</div>
+                <div *ngIf="cfeeRate.hasError('min')">Enter at least a value of {{ minRequiredFee }} sat/vB! If you are really sure that you want a fee rate that is lower than the recommended rate, scroll down and deactivate the check.</div>
               </div>
 
             </div>
@@ -105,7 +104,8 @@ <h2 class="mt-2 mb-4">Mint a CAT-21 Ordinal</h2>
                 We automatically select your largest UTXO, but feel free to select a different one if you like:
               </p>
 
-              <a *ngIf="minRequiredFee != 1" href="javascript:void(0)" (click)="updateMinRequiredFee(1)">⚠️ Click here to disable the required fee rate check (do you know what you are doing!?)</a>
+              <a *ngIf="minRequiredFee != 0" href="javascript:void(0)" (click)="updateMinRequiredFee(0)">⚠️ Click here to disable the required fee rate check.</a>
+              <span *ngIf="minRequiredFee == 0">⚠️ Required fee rate check disabled. You can enter any value!</span>
 
               <div class="mt-4 pl-2 pr-2 pt-1 pb-1 shape-border" *ngFor="let x of paymentOutputs$ | async"
                 [ngClass]="{ selected: x === this.selectedPaymentOutput }">

frontend/src/app/components/_ordpool/cat21-mint/cat21-mint.component.ts

@@ -121,7 +121,8 @@ export class Cat21MintComponent implements OnInit {
               BigInt(0)
             );
 
-            const transactionFee = BigInt(simulation1.vsize * feeRate);
+            // feeRate can be fractional (e.g. 0.5 sat/vB), so we ceil to avoid BigInt crash
+            const transactionFee = BigInt(Math.ceil(simulation1.vsize * feeRate));
 
             // simulate the transaction again, with exact transactionFee
             const simulation2 = this.cat21Service.simulateTransaction(
@@ -199,7 +200,7 @@ export class Cat21MintComponent implements OnInit {
     this.cfeeRate.setValidators([
       Validators.required,
       Validators.min(this.minRequiredFee),
-      fullNumberValidator()
+      // fullNumberValidator() -- removed: sub-sat fee rates (e.g. 0.5 sat/vB) are valid now
     ]);
 
     if (this.cfeeRate.value < this.minRequiredFee) {