Search: Fix up sidebar search within tags

The way this works is currently very confusing.

1. Open sidebar
2. Click a tag
3. The tag is now the "placeholder" within the Search field and all searches will be within this tag
4. The only way to search within all notes again is to go back to the sidebar and click All Notes

This needs to do... not that. At a first guess, clicking on a sidebar tag should probably fill out the search box with the `tag:` syntax so it's clear what is happening, and the user can clear the search.

No files or tests are named. Start by tracing the sidebar tag click and how it updates the search field; done should make the tag: syntax visible in the field and let the user clear it to search all notes again.

Written by the indexing model from the issue text.