Show, hide, and unhide documents with tag-based visibility controls

Release notes:

• Hide documents from view — 🙈 icon on each card tags the document as "hidden" and fades it out
• "Show 🙈" toggle button — next to page count in all views, reveals hidden cards alongside normal ones
• Unhide (👀) icon — on hidden cards, removes the "hidden" tag and restores normal visibility
• New POST /api/remove-tag endpoint — removes a single tag from a document (CSRF-protected)
• Card action icons restyled — solid colorful emoji (📋 🔖 🙈 ❌) at full opacity, no more faded icons in dark mode
• Docs updated — API_Reference (add-tags + remove-tag endpoints), User_Guide (hiding/unhiding section), README changelog
• Added PII detection for Banking routing numbers.