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/profile.go

@@ -23,6 +23,11 @@ import (
 // is unbounded; auto-provisioned platform names bypass this editor validation).
 const maxDisplayName = 32
 
+// maxDisplayNameSpecials caps the total special characters (the "." / "_" separators —
+// every name rune that is neither a letter nor a space) an editable display name may
+// carry, so a still-well-formed name cannot be made of mostly punctuation (Stage 17).
+const maxDisplayNameSpecials = 5
+
 // maxAwayWindow bounds the daily away window's duration (midnight-wrap aware).
 const maxAwayWindow = 12 * time.Hour
 
@@ -110,6 +115,15 @@ func ValidateDisplayName(raw string) (string, error) {
 	if !displayNameRe.MatchString(name) {
 		return "", fmt.Errorf("%w: display name has an invalid character or layout", ErrInvalidProfile)
 	}
+	specials := 0
+	for _, r := range name {
+		if r != ' ' && !unicode.IsLetter(r) {
+			specials++
+		}
+	}
+	if specials > maxDisplayNameSpecials {
+		return "", fmt.Errorf("%w: display name has more than %d special characters", ErrInvalidProfile, maxDisplayNameSpecials)
+	}
 	return name, nil
 }
 

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) {