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/static/app.css

@@ -744,6 +744,19 @@ button[type="submit"]:focus-visible,
   background: var(--surface-2);
 }
 .dropzone.dragover { border-width: 2px; }
+/* #171: uploaded-file preview replaces the editor area; image fitted inside */
+.file-preview {
+  flex: 1;
+  min-width: 0;
+  min-height: 0;
+  max-width: 100%;
+  max-height: 100%;
+  object-fit: contain;
+  border-radius: 8px;
+}
+.editor-wrap.previewing .editor,
+.editor-wrap.previewing .gutter { display: none; }
+
 .file-chip {
   display: flex; align-items: center; gap: 10px;
   border: 1px solid var(--border); border-radius: var(--radius);