feat(tasks): add optional "none" priority level for tasks without urgency

New tasks default to "none" priority instead of "medium". Tasks with no
priority hide the badge in list and dashboard views, reducing visual noise
for routine items. Includes DB migration v4 and i18n keys (de, en, it).

Closes #15

public/pages/dashboard.js

@@ -167,7 +167,7 @@ function renderUrgentTasks(tasks) {
     const due = formatDueDate(t.due_date);
     return `
       <div class="task-item" data-route="/tasks" role="button" tabindex="0">
-        <div class="task-item__priority task-item__priority--${t.priority}" aria-hidden="true"></div>
+        ${t.priority !== 'none' ? `<div class="task-item__priority task-item__priority--${t.priority}" aria-hidden="true"></div>` : ''}
         <span class="sr-only">${PRIORITY_LABELS()[t.priority] ?? t.priority}</span>
         <div class="task-item__content">
           <div class="task-item__title">${esc(t.title)}</div>