Add configurable context warning alerts (fixes #38)

[vanclute]

Summary

Adds visual threshold warnings to context widgets to help users monitor context window usage and manage long conversations.

Features

• 🟡 Yellow warning at configurable threshold (default: 120k tokens)
• 🔴 Red critical at configurable threshold (default: 140k tokens)
• Applies to Context Length and Context Percentage widgets
• Fully configurable via settings.json

Configuration

{
  "tokenWarnings": {
    "enabled": true,
    "warningThreshold": 120000,
    "criticalThreshold": 140000,
    "showEmojis": true
  }
}

Why Context Warnings?

• Helps stay aware of context window consumption
• Useful for managing long conversations and avoiding auto-compact
• Thresholds are personal preference - defaults (120k/140k) are starting points

Implementation

• Shared utility function for consistent behavior across context widgets
• Clean separation of concerns (settings → utility → widgets)
• Backwards compatible (warnings enabled by default)
• Tested locally with various threshold values

Closes #38