feat(web): apply Otter Orange design system and UI improvements

Adopt the landing page's Otter Orange (#E07832) design system across
the web app, replacing the generic blue (#2563eb) theme. Light mode
uses warm cream/stone neutrals, dark mode uses deep brown tones.

UI changes:
- Remove Quick Actions section from home page
- Add modality filter tabs to file-uploaded view
- Remove colored left-border accents from tool panels and grid cards
- Use standard dropzone for pdf-to-image (custom-results mode)
- Fix PDF document viewer to use ArrayBuffer instead of blob URL
- Fix missing FileImage import in dropzone

apps/web/src/lib/tool-display-modes.ts

@@ -78,7 +78,7 @@ export const TOOL_DISPLAY_MODES: Record<string, DisplayMode> = {
   favicon: "before-after",
   "image-to-pdf": "before-after",
   "optimize-for-web": "before-after",
-  "pdf-to-image": "no-dropzone",
+  "pdf-to-image": "custom-results",
 
   // Adjustments extra
   "replace-color": "before-after",