Watch the pref reach the screen, not just the function

Review pointed out that `dailyGroups` was well covered and the wiring to it
was not: passing the wrong pref from App, binding the checkbox to a
neighbour, or hardcoding `true` in Dailies each shipped with the suite
green. Three of those are now caught by rendering the strip and the settings
panel rather than the function behind them — verified by performing the
mutations, not by reading the code.

The fourth, App handing Dailies the wrong pref, is still uncovered: no test
renders App, and closing it needs a feed and a click library this project
does not have.

Also pins the default. Flipping that one line silently empties Today's
dailies for every existing reader, which is the whole argument of the
comment above it, and nothing was watching — `defaults` is exported so the
value and the upgrade path can both be asserted.

And DATA-MODEL's enumeration of the prefs blob had gone stale; AGENTS.md
names it as the doc to update when a stored key moves.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

src/client/state/usePrefs.ts

@@ -186,7 +186,7 @@ export interface Prefs {
   onboarded: boolean;
 }
 
-function defaults(): Prefs {
+export function defaults(): Prefs {
   return {
     region: guessRegion(),
     hiddenGames: [],