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

lib/smol/link.dart

@@ -0,0 +1,27 @@
+// Deep links (SPEC.md §3): the OS hands the app a URI a QR scanner or a
+// browser produced. Only the smol scheme is ours to interpret, and the
+// native parser decides validity — fumi rejects a smol:// URI without its
+// key, so every link that parses is the self-certifying form.
+
+import "package:smol_mail/smol/address.dart";
+import "package:smol_mail/smol/errors.dart";
+
+class SmolLink {
+  final SmolAddress address;
+  final String source;
+
+  const SmolLink._(this.address, this.source);
+}
+
+/// Parses a smol:// URI. Returns null for other schemes and for anything
+/// the native parser rejects — those are not silent fixes, they are not
+/// for this app to act on.
+SmolLink? parseSmolLink(String uri) {
+  final text = uri.trim();
+  if (!text.startsWith("smol://")) return null;
+  try {
+    return SmolLink._(parseAddress(text), text);
+  } on SmolError {
+    return null;
+  }
+}