Task editor drops the split view for tabs

The task sidebar showed the markdown source and the rendered preview next to each other. On a T-70 card with a table in it, each half was so narrow the text wrapped every few words and neither side was readable. The fix was to show one pane at a time.

What changed

TaskEditor.vue now renders a single full-width pane with two tabs on top: Markdown and Preview. The tab choice sticks in localStorage, so reopening a card lands where you left it. The old side-by-side grid is gone, which also fixes the cramped layout on small screens where the preview used to hide entirely.

A second commit added an "Open in new tab" button that only appears in Preview mode. It writes the rendered HTML into a blank browser tab inside a 720px centered column on a dark background, with the card title as the page title. Long cards are now readable without the sidebar squeezing them.

The checks

vue-tsc --noEmit passes on both commits. Both board tasks (editor-preview-tabs and editor-open-preview-window) moved todo to in-progress to done as the work landed, and the commits are efe5bbc and 796ac04 on main.