Allow dot in usernames per NIP-05 local-part spec

Extend usernameRE to [a-z0-9_.-], preserve dots in SanitizeForUsername,
and add tests for validation, sanitization, and nip05 sync precedence.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/user/nostr_sync_test.go

@@ -17,6 +17,10 @@ func TestSanitizeForUsername(t *testing.T) {
 		{"alice_", "alice"},
 		{"alice--bob", "alice-bob"},
 		{"alice__bob", "alice_bob"},
+		{"alice.bob", "alice.bob"},
+		{"alice..bob", "alice.bob"},
+		{".alice", "alice"},
+		{"alice.", "alice"},
 		{"  ", ""},
 		{"", ""},
 		{"!!!", ""},
@@ -59,6 +63,11 @@ func TestCandidateFromMetadataPrecedence(t *testing.T) {
 			name:  "alice", nip05: "preferred@azzamo.net", dom: domain,
 			want: "preferred",
 		},
+		{
+			desc:  "nip05 local part with dot is preserved",
+			name:  "alice", nip05: "alice.smith@azzamo.net", dom: domain,
+			want: "alice.smith",
+		},
 		{
 			desc:  "nip05 ignored when domain differs",
 			name:  "alice", nip05: "preferred@other.example", dom: domain,