feat: support smol:// QR sharing, deep links and in-dialog scanning

lib/presentation/widgets/qr_code_sheet.dart

@@ -0,0 +1,52 @@
+import "package:flutter/material.dart";
+import "package:qr_flutter/qr_flutter.dart";
+
+import "package:smol_mail/shared/utils/format.dart";
+
+/// Shows [payload] as a scannable QR code in a bottom sheet. Fixed dark
+/// modules on white regardless of theme: contrast is what a scanner needs,
+/// not the palette. Scrolled, because the sheet is height-capped on small
+/// screens.
+Future<void> showQrCodeSheet(BuildContext context, String payload) {
+  return showModalBottomSheet<void>(
+    context: context,
+    builder: (ctx) => SingleChildScrollView(
+      padding: const EdgeInsets.fromLTRB(20, 24, 20, 24),
+      child: Column(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          Container(
+            padding: const EdgeInsets.all(12),
+            color: Colors.white,
+            child: QrImageView(
+              data: payload,
+              version: QrVersions.auto,
+              size: 260,
+              backgroundColor: Colors.white,
+              eyeStyle: const QrEyeStyle(
+                eyeShape: QrEyeShape.square,
+                color: Color.fromRGBO(1, 17, 29, 1),
+              ),
+              dataModuleStyle: const QrDataModuleStyle(
+                dataModuleShape: QrDataModuleShape.square,
+                color: Color.fromRGBO(1, 17, 29, 1),
+              ),
+            ),
+          ),
+          const SizedBox(height: 15),
+          SelectableText(
+            payload,
+            style: Theme.of(ctx).textTheme.bodySmall,
+            textAlign: TextAlign.center,
+          ),
+          const SizedBox(height: 10),
+          TextButton.icon(
+            onPressed: () => copyText(ctx, payload),
+            icon: const Icon(Icons.copy, size: 18),
+            label: const Text("Copy"),
+          ),
+        ],
+      ),
+    ),
+  );
+}