fix(ui): prevent search input from clearing on each keystroke and add dedicated /search page

The debounceSearch() function in files.html called clearFullTextSearch()
when the query was shorter than 2 characters. Since clearFullTextSearch()
sets input.value = '', every single keystroke was immediately erased —
users could paste text but not type.

Fix: debounceSearch now only hides the results panel for short queries
without touching the input value.

Also adds a dedicated /search page with Google-style results showing
content previews (document title, filename, type badges, tag badges,
sender, and OCR text snippets with highlighted matches).

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/base.html

@@ -59,6 +59,7 @@
             <a href="/" class="text-gray-700 hover:text-gray-900">Home</a>
             <a href="/upload" class="text-gray-700 hover:text-gray-900">Upload</a>
             <a href="/files" class="text-gray-700 hover:text-gray-900">Files</a>
+            <a href="/search" class="text-gray-700 hover:text-gray-900">Search</a>
 
             <!-- Admin dropdown (shown only for admin users via JS) -->
             <div id="adminMenuContainer" class="relative hidden">
@@ -150,6 +151,7 @@
               <a href="/" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">Home</a>
               <a href="/upload" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">Upload</a>
               <a href="/files" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">Files</a>
+              <a href="/search" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">Search</a>
 
               <!-- Admin section in mobile menu (shown only for admin users via JS) -->
               <div id="mobileAdminSection" class="hidden">