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/style.css

@@ -8428,3 +8428,60 @@ body.popup-mode .content-area {
   font-size: 0.62rem; color: var(--text-muted); opacity: 0.5;
 }
 .about-copy code { font-family: var(--mono); font-size: 0.58rem; }
+
+/* ── PDF Viewer ── */
+.pdf-viewer-container {
+  display: flex;
+  flex-direction: column;
+  height: 100%;
+}
+.pdf-toolbar {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 8px 16px;
+  background: var(--surface1);
+  border-bottom: 1px solid var(--border);
+  font-size: 0.85rem;
+  flex-shrink: 0;
+}
+.pdf-info {
+  flex: 1;
+  color: var(--text-dim);
+}
+.pdf-iframe {
+  flex: 1;
+  width: 100%;
+  border: none;
+  min-height: 70vh;
+}
+
+/* ── PaneManager — Split View ── */
+.pane-grid {
+  display: grid;
+  height: 100%;
+  overflow: hidden;
+}
+.pane-container {
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
+  position: relative;
+}
+.pane-container.pane-active {
+  outline: 2px solid var(--accent);
+  outline-offset: -2px;
+  z-index: 1;
+}
+.pane-tab-bar { flex-shrink: 0; }
+.pane-content { flex: 1; overflow-y: auto; }
+.pane-resize-handle {
+  background: var(--border);
+  transition: background 150ms;
+  min-width: 4px;
+  min-height: 4px;
+  z-index: 2;
+}
+.pane-resize-handle:hover {
+  background: var(--accent);
+}