Multi-account feature

Markdown toolbar + preview in the composer. New toolbar above the textarea with buttons for bold, italic, link, bulleted list, numbered list, blockquote, inline code, and heading. Toolbar actions wrap the current selection (or insert a placeholder that becomes the next selection — VS Code pattern) and restore the cursor after Preact's re-render via a microtask scheduler.
👁 Preview toggle swaps the textarea for a rendered Markdown pane. Parsing uses snarkdown for the practical Micropub subset (paragraphs, headings, bold/italic, links, lists, code spans, blockquotes) and DOMPurify to neutralize any raw <script> / <iframe> / on*-handlers that might survive — important because the preview renders inside the privileged extension popup.
Lazy-loaded markdown machinery: snarkdown + DOMPurify (~27 kB combined) only download when the user clicks Preview. Cold popup-open cost grew by ~5.75 kB (toolbar component code) instead of the +32 kB that eager-loading would have cost.