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);

internal/web/static/new.js

@@ -230,6 +230,64 @@ function humanSize(n) {
   return n + ' B';
 }
 
+// #171: text-file extensions -> language key for title placeholder
+// convention (e.g. Python.py); fallback Text.
+const LANG_BY_EXT = {
+  py: 'python', go: 'go', js: 'javascript', mjs: 'javascript', ts: 'typescript',
+  tsx: 'typescript', rs: 'rust', c: 'c', h: 'c', cpp: 'cpp', cc: 'cpp', hpp: 'cpp',
+  java: 'java', cs: 'csharp', sh: 'bash', bash: 'bash', sql: 'sql', yml: 'yaml',
+  yaml: 'yaml', json: 'json', html: 'html', htm: 'html', css: 'css', xml: 'xml',
+  php: 'php', rb: 'ruby', pl: 'perl', lua: 'lua', toml: 'toml', ini: 'ini',
+  diff: 'diff', md: 'markdown', txt: 'text', log: 'text',
+};
+const TEXT_EXTS = new Set(Object.keys(LANG_BY_EXT));
+
+function extOf(name) {
+  const i = name.lastIndexOf('.');
+  return i >= 0 ? name.slice(i + 1).toLowerCase() : '';
+}
+
+// #171: when a file is attached it takes over the main editing area.
+// Images render fitted into the editor area; text files load their
+// content into the editor. Title auto-fills only if still blank (house
+// rule: never overwrite a typed title).
+const IMAGE_RE = /^image\//;
+let previewURL = null;
+
+async function showFileInEditor(file) {
+  const wrap = document.querySelector('.editor-wrap');
+  const img = $('file-preview');
+  if (IMAGE_RE.test(file.type)) {
+    if (previewURL) URL.revokeObjectURL(previewURL);
+    previewURL = URL.createObjectURL(file);
+    img.src = previewURL;
+    img.alt = file.name;
+    wrap.classList.add('previewing');
+    img.classList.remove('hidden');
+    return;
+  }
+  wrap.classList.remove('previewing');
+  img.classList.add('hidden');
+  if (!TEXT_EXTS.has(extOf(file.name))) return; // unknown binary: leave editor alone
+  try {
+    const text = await file.text();
+    if (!content.value.trim()) {
+      content.value = text;
+      updateGutter();
+      guessLang();
+    }
+  } catch (e) {}
+}
+
+function clearPreview() {
+  const wrap = document.querySelector('.editor-wrap');
+  const img = $('file-preview');
+  wrap.classList.remove('previewing');
+  img.classList.add('hidden');
+  img.removeAttribute('src');
+  if (previewURL) { URL.revokeObjectURL(previewURL); previewURL = null; }
+}
+
 function setAttachedFile(file) {
   if (!file) return clearAttachedFile();
   if (file.size > MAX_FILE_BYTES) {
@@ -239,6 +297,23 @@ function setAttachedFile(file) {
   attachedFile = file;
   renderFileChip();
   setHidden('file-text-note', false);
+  // #171: title auto-fill — images take the file name; text files use the
+  // language-placeholder convention (e.g. Python.py, fallback Text.txt).
+  // Only when the title is still blank; never overwrite a typed title.
+  if (!$('title').value.trim()) {
+    if (IMAGE_RE.test(file.type)) {
+      $('title').value = file.name;
+    } else {
+      const lang = LANG_BY_EXT[extOf(file.name)];
+      if (lang) {
+        const fn = defaultFilename(lang);
+        if (fn) $('title').value = fn;
+      } else if (TEXT_EXTS.has(extOf(file.name))) {
+        $('title').value = defaultFilename('text') || 'Text.txt';
+      }
+    }
+  }
+  showFileInEditor(file);
 }
 
 function clearAttachedFile() {
@@ -246,6 +321,7 @@ function clearAttachedFile() {
   $('file-input').value = '';
   renderFileChip();
   setHidden('file-text-note', true);
+  clearPreview();
 }
 
 function renderFileChip() {