Feature: add timer

There are times when you forgot to add a timer for a previous work period.

Currently you have to add the timer file manually to the `time-data/` directory, or use the interface to create a new timer file, and then edit it.

For this case it would nice to have a way of adding a timer for a previous time period.

A possible implementation could be to add a `+` button to the reports sidebar which would popup an interface to input the date, start time, end time, and notes

Start by tracing how the existing interface creates timer files and how the reports sidebar is assembled; the issue identifies the `time-data/` directory and the reports sidebar as the relevant entry points. Define the date, start time, end time, and notes inputs, then verify that the new flow creates a timer for a previous period without manual file editing.

Written by the indexing model from the issue text.