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

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+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 }
+})

frontend/test/seriesReader.spec.js

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+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
+import { setActivePinia, createPinia } from 'pinia'
+import {
+  useSeriesReaderStore,
+  currentPageFromScroll,
+  progressPct,
+  clampPage
+} from '../src/stores/seriesReader.js'
+
+function stubFetch(handler) {
+  globalThis.fetch = vi.fn(async (url, init) => {
+    const { status, body } = handler(url, init)
+    return {
+      ok: status >= 200 && status < 300,
+      status, statusText: String(status),
+      text: async () => (body == null ? '' : JSON.stringify(body))
+    }
+  })
+}
+
+const M = [
+  { page_number: 1, top: 0, height: 100 },
+  { page_number: 2, top: 100, height: 100 },
+  { page_number: 3, top: 200, height: 100 }
+]
+
+describe('seriesReader pure helpers', () => {
+  it('currentPageFromScroll: top of doc → page 1', () => {
+    expect(currentPageFromScroll(M, 0, 90)).toBe(1)   // target 30
+  })
+  it('currentPageFromScroll: boundary crosses into page 2', () => {
+    expect(currentPageFromScroll(M, 80, 90)).toBe(2)  // target 110
+  })
+  it('currentPageFromScroll: scrolled past last → last page', () => {
+    expect(currentPageFromScroll(M, 5000, 300)).toBe(3)
+  })
+  it('currentPageFromScroll: empty metrics → 1', () => {
+    expect(currentPageFromScroll([], 0, 90)).toBe(1)
+  })
+  it('progressPct: 0 at top, ~50 mid, clamped 100, zero-guard', () => {
+    expect(progressPct(0, 1000, 500)).toBe(0)
+    expect(progressPct(250, 1000, 500)).toBe(50)
+    expect(progressPct(9999, 1000, 500)).toBe(100)
+    expect(progressPct(0, 100, 100)).toBe(0)   // denom 0 → 0
+  })
+  it('clampPage: below→1, above→total, valid passthrough, NaN→1', () => {
+    expect(clampPage(0, 5)).toBe(1)
+    expect(clampPage(99, 5)).toBe(5)
+    expect(clampPage(3, 5)).toBe(3)
+    expect(clampPage(NaN, 5)).toBe(1)
+    expect(clampPage(2, 0)).toBe(1)
+  })
+})
+
+describe('seriesReader store', () => {
+  beforeEach(() => setActivePinia(createPinia()))
+  afterEach(() => vi.restoreAllMocks())
+
+  it('load() stores series + pages', async () => {
+    const s = useSeriesReaderStore()
+    stubFetch(() => ({
+      status: 200,
+      body: {
+        series: { id: 7, name: 'V' },
+        pages: [{ image_id: 1, page_number: 1, thumbnail_url: 't', image_url: 'i' }]
+      }
+    }))
+    await s.load(7)
+    expect(s.series).toEqual({ id: 7, name: 'V' })
+    expect(s.pages[0].image_url).toBe('i')
+    expect(s.error).toBe(null)
+  })
+
+  it('load() on 404 sets error, no throw', async () => {
+    const s = useSeriesReaderStore()
+    stubFetch(() => ({ status: 404, body: { error: 'not a series' } }))
+    await s.load(9)
+    expect(s.error).toBe('not a series')
+    expect(s.pages).toEqual([])
+  })
+})