feat(image_viewer): add multi-page PDF navigation and fix GPU driver crash

- Render PDF pages on-demand per active page instead of stitching entire document into one giant texture (fixing 22k px Vulkan texture atlas overflow and driver crash).
- Add toolbar navigation buttons (previous, next, page indicator).
- Add keyboard shortcuts (PageUp/PageDown, Left/Right arrows, j/k, Home/End) for page navigation.
- Add status bar metadata for active PDF page index and total page count.

assets/keymaps/default-linux.json

@@ -1582,6 +1582,14 @@
       "ctrl-1": "image_viewer::ZoomToActualSize",
       "ctrl-k r": "editor::RevealInFileManager",
       "ctrl-shift-0": "image_viewer::FitToView",
+      "right": "image_viewer::NextPage",
+      "pagedown": "image_viewer::NextPage",
+      "j": "image_viewer::NextPage",
+      "left": "image_viewer::PreviousPage",
+      "pageup": "image_viewer::PreviousPage",
+      "k": "image_viewer::PreviousPage",
+      "home": "image_viewer::FirstPage",
+      "end": "image_viewer::LastPage"
     },
   },
   {

assets/keymaps/default-macos.json

@@ -1629,6 +1629,14 @@
       "cmd-1": "image_viewer::ZoomToActualSize",
       "cmd-k r": "editor::RevealInFileManager",
       "cmd-shift-0": "image_viewer::FitToView",
+      "right": "image_viewer::NextPage",
+      "pagedown": "image_viewer::NextPage",
+      "j": "image_viewer::NextPage",
+      "left": "image_viewer::PreviousPage",
+      "pageup": "image_viewer::PreviousPage",
+      "k": "image_viewer::PreviousPage",
+      "home": "image_viewer::FirstPage",
+      "end": "image_viewer::LastPage"
     },
   },
   {

assets/keymaps/default-windows.json

@@ -1558,6 +1558,14 @@
       "ctrl-1": "image_viewer::ZoomToActualSize",
       "ctrl-k r": "editor::RevealInFileManager",
       "ctrl-shift-0": "image_viewer::FitToView",
+      "right": "image_viewer::NextPage",
+      "pagedown": "image_viewer::NextPage",
+      "j": "image_viewer::NextPage",
+      "left": "image_viewer::PreviousPage",
+      "pageup": "image_viewer::PreviousPage",
+      "k": "image_viewer::PreviousPage",
+      "home": "image_viewer::FirstPage",
+      "end": "image_viewer::LastPage"
     },
   },
   {