fix(accounts): a display name may not read as another account's

A member kept renaming themselves to a colleague's display name, plus
"(you)", so the CPU/RAM card and every owner list showed two of that person.
A display name is now refused (409 ENAMETAKEN) when, with marks, case,
spaces, punctuation, bracketed parts and look-alike Cyrillic/Greek letters
ignored, it equals another account's display name or username — in
Settings → Account and on the Users page alike. Keeping the name one already
has stays allowed, so an older clash does not block an admin's other edits.

server/lib/auth.js

@@ -20,7 +20,8 @@
  *   admin on the Users page (`server/routes/users.js`), where every change
  *   needs the admin's own password again (`confirmPassword`, throttled).
  *   An account also carries an optional display name and email
- *   (`validateProfile`), which its owner edits in Settings → Account without
+ *   (`validateProfile`; no two accounts may read as one name — `assertNameFree`,
+ *   ENAMETAKEN), which its owner edits in Settings → Account without
  *   that step-up; changing their OWN password there needs the old one and
  *   keeps the login it was changed from (`setPassword`'s `keepToken`).
  * - Two password bars: an admin may GIVE any non-empty password (a create or a
@@ -247,6 +248,47 @@ function validateProfile(body) {
   return out;
 }
 
+// Latin look-alikes from other scripts, so "Vĩ" in Cyrillic still collides.
+const CONFUSABLE = {
+  а: "a", в: "b", е: "e", к: "k", м: "m", н: "h", о: "o", р: "p", с: "c", т: "t",
+  у: "y", х: "x", і: "i", ј: "j", ԁ: "d", ѕ: "s", ӏ: "l", α: "a", β: "b", ε: "e",
+  η: "n", ι: "i", κ: "k", ν: "v", ο: "o", ρ: "p", τ: "t", υ: "u", χ: "x", "ı": "i",
+};
+
+/**
+ * The name two accounts may not share: no marks (Vĩ = Vi), đ = d, any case,
+ * look-alike letters folded, and only letters and digits outside brackets —
+ * so "Nguyễn Ngọc Trí Vĩ (you)" and "nguyen.ngoc-tri vi" are both one name.
+ */
+function nameKey(s) {
+  return String(s || "")
+    .normalize("NFKD")
+    .replace(/\p{M}/gu, "")
+    .toLowerCase()
+    .replace(/\([^)]*\)|\[[^\]]*\]/g, "")
+    .replace(/[đð]/g, "d")
+    .replace(/./gu, (c) => CONFUSABLE[c] ?? c)
+    .replace(/[^\p{L}\p{N}]/gu, "");
+}
+
+/**
+ * Refuse (ENAMETAKEN) a display name that reads as another account's display
+ * name or username, so no one can pose as a colleague. Keeping the name one
+ * already has is always allowed — a clash from before this rule is no reason
+ * to block an admin's unrelated edit.
+ */
+function assertNameFree(userId, fields) {
+  if (!("display_name" in fields) || !fields.display_name) return;
+  const key = nameKey(fields.display_name);
+  if (!key) return;
+  const own = sql("SELECT display_name FROM users WHERE id = ?").get(userId);
+  if (own && own.display_name === fields.display_name) return;
+  const rows = sql("SELECT id, username, display_name FROM users WHERE id != ?").all(userId);
+  if (rows.some((u) => nameKey(u.username) === key || nameKey(u.display_name) === key)) {
+    throw err("ENAMETAKEN", "displayName: another account already uses that name");
+  }
+}
+
 /** Write `validateProfile`'s result for one account. */
 function setProfile(userId, fields) {
   for (const [col, value] of Object.entries(fields)) {
@@ -812,6 +854,8 @@ module.exports = {
   unassignWorkspace,
   ownedWorkspaces,
   validateProfile,
+  nameKey,
+  assertNameFree,
   setProfile,
   validatePassword,
   validateOwnPassword,