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/app.js

@@ -18,6 +18,7 @@ import { initCommandPalette } from './palette.js';
 import { initMobileToolbar } from './mobile-toolbar.js';
 import { initDragDrop } from './dragdrop.js';
 import { initMermaid, setupFocusMode } from './mermaid-viewer.js';
+import PaneManager from './pane-manager.js';
 
 // =========================================================================
 // Initialization — mirrors the original app.js init() ordering
@@ -58,6 +59,7 @@ async function init() {
   UI.FindInPageManager.init();
   UI.ContextMenuManager.init();
   UI.TabManager.init();
+  PaneManager.init();
   initCommandPalette();
   initMobileToolbar();