UI redesign: multi-tool mode, workbench bar animation, PDFium rendering fixes

- Add multi-tool mode support across FileSidebar, FilesModalContext, PageEditorContext,
  WorkbenchBar, and ToolWorkflowContext — adding files stays in pageEditor view and
  auto-selects files; left panel hides/shows on workbench switch
- Animate WorkbenchBar in/out via CSS grid-template-rows trick when files are added/removed;
  suppress initial transition with rAF flag
- Fix PDFium pixel format (RGBA not BGRA) and rotation logic (effective dims already
  post-rotation; counter-rotate when applyRotation=false)
- Fix PageEditor flash of "No PDF files loaded" by checking file context directly
- Redesign AddFileCard: solid border, rounded corners, shadow hover, focus-visible ring
- Show AddFileCard in FileEditor empty state instead of text stack
- Add date grouping helpers (today/yesterday/thisWeek/thisMonth/older) to FileSidebar
- Redesign FilePdfIcon: solid filled with folded corner, red colour
- Simplify NonPdfBanner: remove file-type badge, keep only convert-to-PDF button
- Fix thumbnail precedence in useToolOperation (prefer metadata.thumbnailUrl)
- Fix fileActions stub dispatch to trigger selector re-render after ADD_FILES
- Switch Annotate to useAllFiles and Automate/AutomationRun to useViewScopedFiles

frontend/src/core/components/shared/FileSidebar.css

@@ -82,6 +82,10 @@
   flex-shrink: 0;
 }
 
+.file-sidebar-search-close {
+  cursor: pointer;
+}
+
 .file-sidebar-search-input {
   flex: 1;
   background: transparent;