fix: Fix active template highlight

[yasuharu519]

What's happening

Two different visible behaviors that turn out to be the same underlying issue:

1. Selecting a template in the sidebar does not highlight that template — the active class stays on whatever item was highlighted before.
2. After duplicating a template, the wrong (unrelated) template gets highlighted, and switching to a different one does not move the highlight.

Both come from the same problem: when a template is selected, the internal editingTemplateIndex is updated but the DOM .active class on the template list is never re-synced.

Fix

updateTemplateListActiveState(templateId) already exists in settings-section-ui.ts for exactly this purpose, but it is no longer called anywhere.
Call it from showSettingsSection whenever the templates section is shown with a templateId.
Since showTemplateEditor() always ends with showSettingsSection('templates', editingTemplate.id), this covers all selection paths (click, duplicate, new, delete-then-select-next, import, URL restore, drag-and-drop reorder).

Fixes #440