unify storage of tag suggestions

### Problem you are trying to address with this feature

Because tag suggestions are remembered in local storage, they are machine- and browser-dependent. So suggestions differ from browser to browser. And if cookies and site data are blocked in a browser, they don't work at all.

Further, they are H-mode-dependent (the extension's tag memory lives in a different part of local storage from via's tag memory), which further fragments the experience.

A common experience of my-recently-used tags across AP/extension/via would be wonderful.

### Your solution

Remember recently-used tags in the backend, provide a consistent experience across all client browsers and modes plus activity pages.

Start by tracing how recently-used tag suggestions are stored in local storage for the extension and via, then identify the backend entry point that could own shared tag memory. Done means recent tags are consistent across browsers, H-modes, clients, and activity pages, including when cookies and site data are blocked.

Written by the indexing model from the issue text.