Refactor download URL extraction logic

[TDannhauer]

Summary

Fix smartmobile attachment links to open/download reliably on mobile by avoiding AJAX navigation and handling non-string download_url payloads.

Context / Problem

In smartmobile mode, opening attachments (for example PDFs) could fail with blank/error pages.
Root cause: attachment links were sometimes built from a non-string download_url value (serialized object), producing invalid URLs such as [object Object]. Also, attachment requests should bypass jQuery Mobile AJAX navigation for binary responses.

Changes

vendor/horde/imp/js/smartmobile.js

In showAttachments():

• Force direct navigation for attachment links:
  • rel: 'external'
  • data-ajax: 'false'
• Add robust URL normalization:
  • Use v.download_url when it is a valid string.
  • If not, extract href from existing v.download HTML as fallback.
  • Skip rendering link when no valid URL can be resolved.
• Add a short inline comment documenting why attachments must bypass AJAX navigation.

Why this approach

• Binary payloads (PDF/images/etc.) should be handled via normal HTTP navigation/download, not page-fragment AJAX routing.
• Fallback extraction from v.download keeps compatibility with payload variants where URL objects are serialized unexpectedly.
• Keeps fix localized to smartmobile attachment rendering without changing backend behavior.

Behavioral Impact

• Smartmobile attachment links now resolve to valid URLs even when download_url is not a string.
• PDF and other attachment downloads/views open correctly on mobile.
• Prevents malformed URLs like /imp/[object Object].

Test Plan

• Open an email with PDF attachment in smartmobile mode and verify it opens/downloads.
• Repeat with image and generic binary attachments.
• Confirm attachment links no longer contain [object Object] in Network tab.
• Confirm dynamic mode behavior remains unchanged.

[TDannhauer]

fixes together with PR horde/base#83 issue #37