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/components/files/file-details.tsx

@@ -188,8 +188,8 @@ export function FileDetails({ mobile = false }: FileDetailsProps) {
       {/* Details card */}
       <div className="flex-1">
         <div className="rounded-lg border border-border overflow-hidden">
-          <div className="bg-blue-500/10 border-b border-border px-3 py-2">
-            <h4 className="text-sm font-semibold text-blue-600 dark:text-blue-400">
+          <div className="bg-primary/10 border-b border-border px-3 py-2">
+            <h4 className="text-sm font-semibold text-primary-dark dark:text-primary-light">
               {t.files.fileDetailsHeading}
             </h4>
           </div>