Fix #184: file upload replaces the main editing area on /new

Image files render fitted into the editor area (editor and gutter
hidden); the title auto-fills with the file name. Text files always
load their content into the editor, replacing any text already there,
instead of only filling an empty editor. Title auto-fill still only
applies when the title is blank; a typed title is never overwritten.
Drawing and cropping tools are noted in the issue as possible future
work and are intentionally not implemented here.

internal/web/static/new.js

@@ -280,11 +280,12 @@ async function showFileInEditor(file) {
   if (!TEXT_EXTS.has(extOf(file.name))) return; // unknown binary: leave editor alone
   try {
     const text = await file.text();
-    if (!content.value.trim()) {
+    // #184: the file replaces the main editing area, so the text always
+    // loads over whatever was in the editor (same rule as images, which
+    // hide the editor entirely).
     content.value = text;
     updateGutter();
     guessLang();
-    }
   } catch (e) {}
 }
 
@@ -306,9 +307,10 @@ function setAttachedFile(file) {
   attachedFile = file;
   renderFileChip();
   setHidden('file-text-note', false);
-  // #171: title auto-fill — images take the file name; text files use the
-  // language-placeholder convention (e.g. Python.py, fallback Text.txt).
-  // Only when the title is still blank; never overwrite a typed title.
+  // #171/#184: title auto-fill — images take the file name; text files
+  // use the language-placeholder convention (e.g. Python.py, fallback
+  // Text.txt). Only when the title is still blank; never overwrite a
+  // typed title.
   if (!$('title').value.trim()) {
     if (IMAGE_RE.test(file.type)) {
       $('title').value = file.name;