Fix pre-release issues with attachment viewer (#12244)

* Fix translation issues for "FIT" and "New Attachment" in attachment editor

* Fix markdown preview persistence and enable external links in attachment editor

* Update preview panel if manually moved from collapsed position

* Match edit view scroll position (by percentage) when changed. This ensures the preview remains in relative sync with the edited document, for example when a large amount of HTML reduces down to a short preview document.

* Fix default preview size to be half the width of the edit widget.

* Set tab stop to 10 and remove base ui file

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Jonathan White <support@dmapps.us>

share/translations/keepassxc_en.ts

@@ -4110,6 +4110,10 @@ Would you like to overwrite the existing attachment?</source>
         <source>Save…</source>
         <translation type="unfinished"></translation>
     </message>
+    <message>
+        <source>New Attachment</source>
+        <translation type="unfinished"></translation>
+    </message>
 </context>
 <context>
     <name>EntryAttributesModel</name>
@@ -4679,6 +4683,10 @@ You can enable the DuckDuckGo website icon service in the security section of th
         <source>Zoom:</source>
         <translation type="unfinished"></translation>
     </message>
+    <message>
+        <source>Fit</source>
+        <translation type="unfinished"></translation>
+    </message>
 </context>
 <context>
     <name>ImportWizard</name>
@@ -9261,10 +9269,6 @@ This option is deprecated, use --set-key-file instead.</source>
         <comment>TOTP</comment>
         <translation type="unfinished"></translation>
     </message>
-    <message>
-        <source>Fit</source>
-        <translation type="unfinished"></translation>
-    </message>
     <message numerus="yes">
         <source>%1 character(s)</source>
         <translation type="unfinished">