docs(helpers): explain djb2 seed constant in stringHash

frontend/src/helpers/stringHash.ts

@@ -1,6 +1,8 @@
 // Deterministic non-cryptographic string hash (djb2 variant).
 // Used for stable pseudo-random selection keyed on date + user + bucket.
 export function stringHash(input: string): number {
+	// 5381 is the canonical djb2 seed — a prime that empirically yields a good
+	// distribution when combined with the `hash * 33 + c` step below.
 	let hash = 5381
 	for (let i = 0; i < input.length; i++) {
 		// hash * 33 + char, kept in 32-bit range via `| 0`.