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).

ui/src/lib/profileValidation.ts

@@ -5,6 +5,10 @@
 /** maxDisplayName caps the editable display name in runes. */
 export const maxDisplayName = 32;
 
+/** maxDisplayNameSpecials caps the total special characters (the "." / "_" separators — every
+ *  rune that is neither a letter nor a space) a display name may carry. Mirrors the Go rule. */
+export const maxDisplayNameSpecials = 5;
+
 /** maxAwayMinutes bounds the daily away window's length (12 h). */
 export const maxAwayMinutes = 12 * 60;
 
@@ -17,7 +21,10 @@ const displayNameRe = /^\p{L}+(?:(?:[._] ?| )\p{L}+)*\.?$/u;
 /** displayNameError returns true when the trimmed name is a valid display name. */
 export function validDisplayName(raw: string): boolean {
   const name = raw.trim();
-  return name.length > 0 && [...name].length <= maxDisplayName && displayNameRe.test(name);
+  const chars = [...name];
+  if (name.length === 0 || chars.length > maxDisplayName || !displayNameRe.test(name)) return false;
+  const specials = chars.filter((c) => c !== ' ' && !/\p{L}/u.test(c)).length;
+  return specials <= maxDisplayNameSpecials;
 }
 
 // A pragmatic email check (the backend re-validates with net/mail). Rejects spaces