Task/Issue URL: https://app.asana.com/1/137249556945/task/1213892682193779

Description

Renders PDF files inline in the browser tab instead of triggering a file download, using AndroidX pdf-viewer-fragment.
When the WebView intercepts a PDF download (application/pdf MIME type or .pdf URL), the file is downloaded to internal cache, validated with %PDF- magic bytes, and displayed in a themed PDF viewer overlaid on the browser tab.

Steps to test this PR

Inline PDF rendering

• Navigate to a URL that serves a PDF (e.g. search for "sample pdf" on DuckDuckGo and tap a .pdf link)
• Verify the omnibar shows a loading progress bar while the PDF downloads
• Verify the PDF renders inline in the browser tab (not a file download prompt)
• Verify you can scroll through multi-page PDFs
• Press back — verify it dismisses the PDF and shows the previous web page

Fallback to download

• On a device below Android 12, verify all PDFs trigger standard download

Cancellation on navigation

• Start loading a large PDF
• While the progress bar is still showing, tap the omnibar and navigate to a different URL
• Verify the PDF does NOT appear after navigation — the download was cancelled (PDF download cancelled visible in logcat)

Process death recovery

• Open a PDF inline, then kill the app process
• Restore the app — press back and verify it navigates home (instead of showing a blank screen)

Feature flag off

• Disable the pdfViewer feature flag
• Verify PDF links trigger standard file download

UI changes