Add view widget to add an activity

It should be possible to add an activity (ideally by category, but if it was easier to be forced to add title that matches current rules that's also fine) from the webui portal. E.g. when you take a meeting on your phone but don't have sync fully done, etc.

This is differnet to a watcher that asks you, etc.

We already support editing events, so adding them shouldn't introduce new mutability where it was previously readonly (because it wasn't).

Alternatively the current stopwatch UI could be extended to not only start an event but to also enable adding one in the past.
If that route was chosen, we should make it show up in activities tho.

Start by reading the existing event-editing flow in the web UI and the current stopwatch UI, then inspect how activities are displayed. Decide whether the new entry point belongs in the portal or stopwatch flow; done means a user can add a past activity, with category or a title meeting current rules, and see it in activities.

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