Move randomFrom helper out of app.svelte.js

[silverbucket]

`src/lib/stores/app.svelte.js:13` defines a one-line `randomFrom(arr)` helper at the top of the store. It's only used a few times, but `utils.js` is the proper home for it (it sits next to `uid`, `clone`, etc.) and one-liners proliferating at the top of stores is a smell.

Fix: move to `utils.js` and import.

(From comprehensive review, finding B8.)

[silverbucket]

Triage

Still applicable. `randomFrom` is defined at `src/lib/stores/app.svelte.js:13` and used four times (lines 1053, 1078, 1090, 1103) — all for the toast-message variation.

Importance: 3 (low). Purely organisational. `utils.js` already houses `uid`, `clone`, `nowIso`, etc. — `randomFrom` belongs there. One line move + one import update.

If the toast-variation pattern grows, consider a `pickRandomToast(messages)` helper — but that's speculative and the issue rightly stays focused on the simple move.