Allow manually adding time entries

[grmbyrn]

Problem

If a user forgets to start the timer, there is no way to log time
retroactively. This is a common real-world scenario.

Proposed Solution

Add a "Log time manually" form that lets the user specify a client,
date, start time, and end time (or duration) without using the live timer.

Acceptance Criteria

• A "Add entry" button is visible on the timer/log page
• Form accepts: client, date, start time, end time or duration
• Entry is saved via POST /api/timer using the same model as live-tracked entries
• Manually added entries appear in the same list as timer entries

Labels

enhancement, high-priority