docs: improve README and product positioning

[pando85]

Summary

Improve the README so it reflects the current program rather than presenting Timer as only a simple countdown alarm.

Changes

• clarify the project positioning as a terminal countdown timer and stopwatch
• add a concise command-oriented introduction with the main use cases
• document flexible duration syntax and target clock times
• document loop, silence, terminal bell, and stopwatch usage
• document stopwatch controls
• reorganize alarm documentation around normal audio, PC speaker, and terminal bell
• simplify and clarify installation instructions
• make the README emphasize Timer's small, configuration-free scope without overselling it

Motivation

The existing README undersells several of Timer's strongest features, particularly target clock times (timer 14:30), the adaptive display, terminal title updates, and the stopwatch. This makes the project look closer to a thin wrapper around sleep than the actual interactive terminal utility it has become.

This PR is intentionally documentation-only and does not change program behavior.