feat: add idle timeout notifications for TUI chat

[linw1995]

Summary

Implements idle timeout notification feature for TUI chat as described in #162.

Changes

• IdleTracker module: New module to track user inactivity and trigger periodic notifications
• Notification config refactor:
  • Replaced only_when_unfocused boolean with flexible NotificationWhen enum (Focused/Unfocused/Always)
  • Added granular notification rules for different events (reply_ready, action_required)
• Idle notification settings: Configurable timeout (default 5min), max notifications (default 1), and interval (default 1min)
• Chat component integration: Polls idle tracker and sends reminders when response is needed

Key Features

✅ Idle timeout detection with configurable duration
✅ Periodic reminders with max limit
✅ Resets on user activity (keyboard input)
✅ Focus-aware (Focused/Unfocused/Always modes)
✅ Backward compatible (disabled by default)

Default Config

[ui.notifications]
enabled = true

[ui.notifications.reply_ready]
enabled = true
when = "unfocused"  # focused, unfocused, always

[ui.notifications.action_required]
enabled = true
when = "unfocused"

[ui.notifications.idle]
enabled = true
timeout_seconds = 300    # 5 minutes
max_notifications = 1
interval_seconds = 60    # 1 minute
when = "always"

Closes #162

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.55556% with 55 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.08%. Comparing base (c3970cb) to head (d4cb347).
⚠️ Report is 4 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/coco-tui/src/components/chat.rs	0.00%	51 Missing ⚠️
crates/coco-tui/src/idle_tracker.rs	95.78%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #167      +/-   ##
==========================================
+ Coverage   47.59%   48.08%   +0.49%     
==========================================
  Files         100      105       +5     
  Lines       27316    27896     +580     
==========================================
+ Hits        13000    13413     +413     
- Misses      14316    14483     +167     

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