Stage 17: cap display-name special characters at 5 (ui + backend)

display_name validation gains a rule: at most 5 special characters — the '.' / '_'
punctuation (spaces, which separate words, don't count) — so a still-well-formed name
can't be mostly punctuation. Mirrored in the Go ValidateDisplayName and the UI
validDisplayName; both unit-tested (5 ok, 6 rejected, 'J. R. R. Tolkien' ok). Docs:
FUNCTIONAL (+ _ru).

backend/internal/account/validate_test.go

@@ -27,6 +27,9 @@ func TestValidateDisplayName(t *testing.T) {
 		"digit rejected":      {"Name2", "", false},
 		"blank":               {"   ", "", false},
 		"too long":            {strings.Repeat("a", 33), "", false},
+		"five specials ok":    {"a.a.a.a.a.a", "a.a.a.a.a.a", true},           // 5 dots
+		"six specials":        {"a.a.a.a.a.a.a", "", false},                   // 6 dots
+		"initials spaces ok":  {"J. R. R. Tolkien", "J. R. R. Tolkien", true}, // 3 dots; spaces don't count
 	}
 	for name, tc := range cases {
 		t.Run(name, func(t *testing.T) {