file upload preview in editor on /new (#171)

- images render fitted into the editor area (object-fit contain), editor hidden
- text files load content into the editor; title placeholder convention (Python.py, Text.txt fallback)
- title auto-fills only when blank; never overwrites a typed title
- external JS + CSS classes only, CSP-safe (no inline styles/scripts)

internal/web/templates/new.html

@@ -26,6 +26,7 @@
     <div class="float editor-wrap">
       <div class="gutter" id="gutter">1</div>
       <textarea class="editor" id="content" placeholder="Paste your code, text, or notes here…" spellcheck="false"></textarea>
+      <img id="file-preview" class="file-preview hidden" alt="File preview">
     </div>
     <div class="created-banner" id="created"></div>
     <div class="actionbar">