feat(reader): in-book search for PDFs

PDFs have fully searchable text (pdf.js text layer) — the previous
reflowable-only gate existed only because foliate's generic search
needs DOM documents that PDF sections don't provide. This adds a PDF
pipeline alongside it:

- Fork d065495 exposes the pdf.js document proxy as book.pdf so the
  host can drive text extraction directly.
- New web/src/reader/pdf-search.ts: extractPdfPages() pulls each
  page's textContent with item geometry (progress-reported, cached
  after first search). PDF text items often omit inter-word spaces
  (gaps are positional), so pages are joined gap-aware — baseline
  changes, hasEOL, or horizontal gaps past a font-size threshold
  become spaces — recording a char→item map. searchPdfPages() does
  case-insensitive matching over the joined text and maps each hit
  back to the page-fraction rects of the items it spans, with
  ellipsized pre/match/post excerpts. Pure functions, unit-sanity
  checked (cross-item 'brave new' → two rects).
- runSearch branches: EPUB keeps foliate's DOM search; PDFs search
  the extracted pages, group hits per page ('Page 12'), and render
  on-page hit rectangles through the existing fraction-rect overlay
  (addRectAnnotation) — which re-render automatically when pages
  revisit, same as highlights. Clearing the query removes them.
- Results navigate by page index; the 🔍 button and '/' shortcut now
  appear for PDFs too (comics remain without searchable text).

web/src/reader/pdf-search.ts

@@ -0,0 +1,163 @@
+// PDF in-book search: text extraction with item geometry, and a matcher
+// that maps hits back to page-fraction rects for the overlay renderer.
+//
+// pdf.js text items carry positional data (transform/width/height in PDF
+// units at scale 1) but their strings often omit inter-word spaces — gaps
+// are positional. Pages are therefore joined gap-aware, with a char→item
+// map so each match can be covered by the rects of the items it spans.
+
+export interface PdfSearchItem {
+  /** page-fraction rect of this text item */
+  x: number;
+  y: number;
+  w: number;
+  h: number;
+  /** char offset of this item's text within the page string */
+  start: number;
+  length: number;
+}
+
+export interface PdfPageText {
+  index: number;
+  /** normalized, gap-joined page text (lowercased by the matcher) */
+  text: string;
+  items: PdfSearchItem[];
+}
+
+export interface PdfSearchHit {
+  page: number;
+  rects: number[][];
+  pre: string;
+  match: string;
+  post: string;
+}
+
+interface RawItem {
+  str: string;
+  transform: number[];
+  width: number;
+  height: number;
+  hasEOL: boolean;
+}
+
+/** Join one page's text items into a searchable string + item map. */
+export function buildPageText(
+  rawItems: RawItem[],
+  viewportWidth: number,
+  viewportHeight: number,
+  index: number,
+): PdfPageText {
+  const vw = viewportWidth || 1;
+  const vh = viewportHeight || 1;
+  let text = "";
+  const items: PdfSearchItem[] = [];
+
+  let prevRight: number | null = null;
+  let prevBaseline: number | null = null;
+
+  for (const item of rawItems) {
+    if (!item.str) continue;
+    const t = item.transform ?? [1, 0, 0, 1, 0, 0];
+    const baseline = t[5] ?? 0;
+    const x = t[4] ?? 0;
+    const size =
+      Math.abs(item.height) || Math.abs(t[3]) || Math.abs(t[0]) || 10;
+    const w = Math.abs(item.width) || 0;
+    const h = size;
+
+    let sep = "";
+    if (text && !text.endsWith(" ") && prevRight != null) {
+      const newLine =
+        item.hasEOL ||
+        prevBaseline == null ||
+        Math.abs(baseline - prevBaseline) > size * 0.5;
+      const gap = x - prevRight;
+      if (newLine || gap > size * 0.2) sep = " ";
+    }
+
+    const s = item.str.replace(/\s+/g, " ");
+    const start = text.length + sep.length;
+    text += sep + s;
+
+    items.push({
+      x: x / vw,
+      y: (vh - baseline - h) / vh,
+      w: w / vw,
+      h: h / vh,
+      start,
+      length: s.length,
+    });
+
+    prevRight = x + w;
+    prevBaseline = baseline;
+  }
+
+  return { index, text: text.trimStart(), items };
+}
+
+/** Extract all pages of a PDF via pdf.js, reporting progress 0..1. */
+export async function extractPdfPages(
+  pdf: any,
+  onProgress?: (fraction: number) => void,
+): Promise<PdfPageText[]> {
+  const pages: PdfPageText[] = [];
+  const num = pdf.numPages as number;
+  for (let i = 0; i < num; i++) {
+    const page = await pdf.getPage(i + 1);
+    const viewport = page.getViewport({ scale: 1 });
+    const tc = await page.getTextContent();
+    pages.push(
+      buildPageText(tc.items, viewport.width, viewport.height, i),
+    );
+    onProgress?.((i + 1) / num);
+  }
+  return pages;
+}
+
+const CONTEXT = 60;
+
+/**
+ * Case-insensitive search over extracted pages. Returns hits grouped in
+ * page order; each hit carries the page-fraction rects of the items it
+ * spans (capped to keep pathological fills cheap) plus a trimmed excerpt.
+ */
+export function searchPdfPages(
+  pages: PdfPageText[],
+  query: string,
+  locales = "en",
+): PdfSearchHit[] {
+  const needle = query.toLocaleLowerCase(locales).replace(/\s+/g, " ").trim();
+  if (!needle) return [];
+  const hits: PdfSearchHit[] = [];
+
+  for (const page of pages) {
+    const haystack = page.text.toLocaleLowerCase(locales);
+    let from = 0;
+    for (;;) {
+      const s = haystack.indexOf(needle, from);
+      if (s === -1) break;
+      const e = s + needle.length;
+      from = s + Math.max(1, needle.length);
+
+      const rects: number[][] = [];
+      for (const it of page.items) {
+        if (it.length <= 0) continue;
+        if (it.start + it.length <= s || it.start >= e) continue;
+        if (rects.length >= 12) break;
+        rects.push([it.x, it.y, it.w, it.h]);
+      }
+      if (!rects.length) continue;
+
+      const pre = page.text.slice(Math.max(0, s - CONTEXT), s);
+      const post = page.text.slice(e, e + CONTEXT);
+      hits.push({
+        page: page.index,
+        rects,
+        pre: (s > CONTEXT ? "…" : "") + pre.trimStart(),
+        match: page.text.slice(s, e),
+        post: post.trimEnd() + (page.text.length > e + CONTEXT ? "…" : ""),
+      });
+    }
+  }
+  return hits;
+}