feat(reader): seriesReader store + pure scroll/progress/clamp helpers

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/stores/seriesReader.js

@@ -0,0 +1,56 @@
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+import { useApi } from '../composables/useApi.js'
+
+// IR reader.js "viewport-center-in-third" rule, pure for unit tests.
+// metrics: [{page_number, top, height}] in page order.
+export function currentPageFromScroll(metrics, scrollTop, viewportH) {
+  if (!metrics.length) return 1
+  const target = scrollTop + viewportH / 3
+  let cur = metrics[0].page_number
+  for (const m of metrics) {
+    if (target >= m.top) cur = m.page_number
+  }
+  return cur
+}
+
+export function progressPct(scrollTop, scrollHeight, clientHeight) {
+  const denom = scrollHeight - clientHeight
+  if (denom <= 0) return 0
+  const pct = (scrollTop / denom) * 100
+  return Math.min(100, Math.max(0, pct))
+}
+
+export function clampPage(n, total) {
+  if (!total || total < 1) return 1
+  if (!Number.isFinite(n) || n < 1) return 1
+  if (n > total) return total
+  return Math.floor(n)
+}
+
+export const useSeriesReaderStore = defineStore('seriesReader', () => {
+  const api = useApi()
+
+  const series = ref(null)
+  const pages = ref([])
+  const loading = ref(false)
+  const error = ref(null)
+
+  async function load(tagId) {
+    loading.value = true
+    error.value = null
+    try {
+      const body = await api.get(`/api/series/${tagId}/pages`)
+      series.value = body.series
+      pages.value = body.pages
+    } catch (e) {
+      error.value = e.message
+      series.value = null
+      pages.value = []
+    } finally {
+      loading.value = false
+    }
+  }
+
+  return { series, pages, loading, error, load }
+})