feat: add Timer domain for Home Assistant timer entities

[Faerkeren]

Summary

• Add Timer domain class (src/haclient/domains/timer.py) supporting HA timer entities (timer.<name>)
• Properties: is_active, is_paused, is_idle, duration, remaining, finishes_at
• Actions: start(duration=), pause(), cancel(), finish(), change(duration=)
• Listener decorators: on_start(), on_pause(), on_idle()
• Register timer() accessor on HAClient

Usage

async with HAClient("http://ha:8123", token="...") as ha:
    t = ha.timer("my_timer")
    await t.start(duration="00:05:00")
    
    @t.on_idle
    def finished(old, new):
        print("Timer done!")

All 131 tests pass. Coverage: 95.07%.