feat: let users change their auto-generated username in Settings

Usernames are auto-assigned at signup (shipped earlier this session)
but there was still no way to change one. Adds a Username field to
Settings, validated client- and server-side against the same
3-20-char lowercase/digits/underscore pattern used for generation,
with a 409 on collision (excluding the user's own current username).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/lib/changelog.ts

@@ -1,5 +1,5 @@
 // Mirrors CHANGELOG.md at the repo root — update both together.
-export const APP_VERSION = "0.13.1";
+export const APP_VERSION = "0.14.0";
 
 export type ChangelogEntry = {
   version: string;
@@ -11,6 +11,13 @@ export type ChangelogEntry = {
 };
 
 export const CHANGELOG: ChangelogEntry[] = [
+  {
+    version: "0.14.0",
+    date: "2026-07-13 21:47",
+    added: [
+      "**Change your username** in Settings — previously auto-assigned at signup with no way to change it.",
+    ],
+  },
   {
     version: "0.13.1",
     date: "2026-07-13 17:25",

apps/web/lib/username.ts

@@ -1,5 +1,7 @@
 import { db, users, eq } from "@epicure/db";
 
+export const USERNAME_PATTERN = /^[a-z0-9_]{3,20}$/;
+
 /** Lowercase alnum-and-underscore slug, at least 3 chars, at most 20. */
 function slugify(seed: string): string {
   const base = seed
@@ -31,3 +33,12 @@ export async function generateUniqueUsername(seed: string): Promise<string> {
     candidate = `${base}${suffix}`.slice(0, 20);
   }
 }
+
+/** True if `candidate` belongs to some other user — a user re-saving their own current username is not a conflict. */
+export async function isUsernameTaken(candidate: string, excludeUserId: string): Promise<boolean> {
+  const existing = await db.query.users.findFirst({
+    where: eq(users.username, candidate),
+    columns: { id: true },
+  });
+  return !!existing && existing.id !== excludeUserId;
+}