fix: stray "0" shown for steps with no timer (v0.52.1)

`{step.timerSeconds && (...)}` renders the literal 0 when timerSeconds is
the number 0 rather than null/undefined — only false/null/undefined skip
rendering in JSX, a bare falsy number doesn't. Some steps ended up with
timerSeconds stored as 0 instead of null (e.g. AI generation filling in a
"default" value for an optional field rather than omitting it), which
this guard then rendered as a bare "0" instead of hiding the timer.

Fixed at all 4 render sites (recipe page, cook mode, both print views) by
coercing to a real boolean (`!!step.timerSeconds`) — makes a stored 0
behave identically to null everywhere it's displayed.

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "epicure",
-  "version": "0.52.0",
+  "version": "0.52.1",
   "private": true,
   "scripts": {
     "dev": "pnpm --filter web dev",