feat: add content_preview helper and apply rendered content across templates

Introduce a new `content_preview()` function in `rendering.py` that strips HTML from rendered content and truncates to a given length with an ellipsis. Add a corresponding `plainText()` and `preview()` method to the `ContentRenderer` JS class. Register `content_preview` as a Jinja2 global in `templating.py`. Update multiple templates (`feed.html`, `gists.html`, `landing.html`, `messages.html`, `news.html`, `notifications.html`, `post.html`, `profile.html`) to use `render_title`, `render_content`, or `content_preview` instead of raw string slicing, ensuring consistent rendering of rich text (e.g., Markdown, HTML) in previews, descriptions, bios, and notification messages. Also fix the conversation preview in `MessagesLayout.js` to use the new JS preview method.

devplacepy/static/js/ContentRenderer.js

@@ -43,6 +43,25 @@ export class ContentRenderer {
         return html;
     }
 
+    plainText(text) {
+        if (!text) return "";
+        let html;
+        try {
+            html = this.render(text);
+        } catch (err) {
+            return String(text).replace(/\s+/g, " ").trim();
+        }
+        const div = document.createElement("div");
+        div.innerHTML = html;
+        return (div.textContent || "").replace(/\s+/g, " ").trim();
+    }
+
+    preview(text, length = 60) {
+        const plain = this.plainText(text);
+        if (plain.length <= length) return plain;
+        return plain.slice(0, length).replace(/\s+$/, "") + "...";
+    }
+
     escapeHtml(text) {
         const div = document.createElement("div");
         div.textContent = text;

devplacepy/static/js/MessagesLayout.js

@@ -457,7 +457,7 @@ export class MessagesLayout {
         const item = document.querySelector(`.conversation-item[data-conv-uid="${partnerUid}"]`);
         if (!item) return;
         const preview = item.querySelector(".conversation-preview");
-        if (preview) preview.textContent = frame.content.slice(0, 60);
+        if (preview) preview.textContent = contentRenderer.preview(frame.content, 60);
         const dot = item.querySelector(".conversation-unread-dot");
         if (dot) {
             dot.hidden = mine || partnerUid === this.otherUid;