feat(06-01): add useReadingTime composable fallback (200 wpm)

- Pure synchronous helper returning minutes (>= 1) from either a pre-computed
  word count (number) or raw text (string, tokenized on whitespace).
- Client-side safety net when `article.minutes` isn't yet populated
  (e.g., dev hot-reload before the Nitro hook re-parsed). Source of truth
  remains the Nitro `content:file:afterParse` hook (D-19).
- Same 200 wpm formula as server-side hook — ensures listing ↔ article parity.
- Auto-imported by Nuxt thanks to `use*` naming convention.

app/composables/useReadingTime.ts

@@ -0,0 +1,17 @@
+/**
+ * Fallback reading-time helper when `article.minutes` is not available
+ * (e.g., dev hot-reload before the Nitro hook has re-parsed).
+ *
+ * Source of truth = server/plugins/reading-time.ts + content.config.ts schema.
+ * This is only a client-side safety net (per D-19).
+ *
+ * @param wordCountOrText number (word count already computed) OR string (raw text to tokenize)
+ * @returns minutes (>= 1), rounded up, using 200 words per minute
+ */
+export function useReadingTime(wordCountOrText: number | string): number {
+  if (typeof wordCountOrText === 'number') {
+    return Math.max(1, Math.ceil(wordCountOrText / 200))
+  }
+  const count = wordCountOrText.trim().split(/\s+/).filter(Boolean).length
+  return Math.max(1, Math.ceil(count / 200))
+}