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/model_test.go

@@ -10,9 +10,12 @@ func TestValidateUsername(t *testing.T) {
 		{"alice", true},
 		{"al-ice_42", true},
 		{"a", true},
+		{"alice.bob", true},
+		{"alice.smith.42", true},
 		{"", false},
 		{"-alice", false},
 		{"_alice", false},
+		{".alice", false},
 		{"thisusernameiswaytoolongtobevalid12345", false},
 		{"admin", false},
 	}