fix(settings): keep Steam ID as string and store the new token

Number("76561198005679788") is 76561198005679790 in JavaScript, so the wrong
Steam ID was saved. The Steam ID now stays a 17-digit string. Theme and
language saves no longer send steamId 0. After a save the new token from Auth
replaces the old one.

Add Vitest and run it in CI.

src/utils/validation.js

@@ -15,14 +15,12 @@ const inviteCodeField = z
     .string()
     .min(1, 'VALIDATION.INVITE_CODE_REQUIRED');
 
-// Backend stores steamId as a primitive long; Steam IDs are positive 64-bit integers.
+// A SteamID64 has 17 digits. Keep it a string: a JS number loses precision above 2^53.
 const steamIdField = z
-    .union([z.string(), z.number()])
-    .transform((v) => (typeof v === 'string' ? v.trim() : String(v)))
-    .refine((v) => v.length > 0, {message: 'VALIDATION.STEAM_ID_REQUIRED'})
-    .refine((v) => /^\d+$/.test(v), {message: 'VALIDATION.STEAM_ID_INVALID'})
-    .transform((v) => Number(v))
-    .refine((n) => Number.isFinite(n) && n > 0, {message: 'VALIDATION.STEAM_ID_INVALID'});
+    .string()
+    .trim()
+    .min(1, 'VALIDATION.STEAM_ID_REQUIRED')
+    .regex(/^\d{17}$/, 'VALIDATION.STEAM_ID_INVALID');
 
 export const registrationSchema = z
     .object({