feat: PDF support + Split View (P1 roadmap items)

#74 — Support PDF complet:
- backend/pdf_reader.py — pymupdf/pypdf text extraction
- .pdf added to SUPPORTED_EXTENSIONS in indexer.py
- PDF-aware file view API endpoint (metadata, text preview)
- Streaming endpoint /api/file/{vault}/pdf/stream
- Frontend PDF viewer with iframe, toolbar, download
- CSS for PDF viewer container

#75 — Split View (PaneManager):
- New module frontend/js/pane-manager.js — ES module
- 1-4 pane CSS grid with resize handles
- Split right/down via keyboard (Ctrl+Alt+\) or PaneManager API
- Persistence in localStorage (obsigate-panes)
- Collapse to single pane, per-pane active state

285 tests passent

frontend/js/viewer.js

@@ -402,6 +402,27 @@ async function renderBacklinksPanel(vault, path, container) {
 export function renderFile(data) {
   const area = document.getElementById("content-area");
 
+  // Handle PDF files — render in iframe
+  if (data.is_pdf) {
+    const pdfUrl = `/api/file/${encodeURIComponent(data.vault)}/pdf/stream?path=${encodeURIComponent(data.path)}`;
+    const pages = data.pdf_metadata && data.pdf_metadata.pages ? data.pdf_metadata.pages : '?';
+    area.innerHTML = `
+      <div class="pdf-viewer-container">
+        <div class="pdf-toolbar">
+          <span class="pdf-info">PDF — ${pages} pages</span>
+          <button class="btn-action" onclick="window.open('${pdfUrl}', '_blank')">
+            <i data-lucide="external-link" style="width:14px;height:14px"></i> Plein écran
+          </button>
+          <button class="btn-action" onclick="window.open('/api/file/${encodeURIComponent(data.vault)}/download?path=${encodeURIComponent(data.path)}', '_blank')">
+            <i data-lucide="download" style="width:14px;height:14px"></i> Télécharger
+          </button>
+        </div>
+        <iframe src="${pdfUrl}" class="pdf-iframe" title="${escapeHtml(data.title)}"></iframe>
+      </div>`;
+    lucide.createIcons();
+    return;
+  }
+
   // Handle unsupported (binary) files
   if (data.unsupported) {
     const sizeStr = data.size_bytes