Optimize chat message storage with circular buffer implementation

[BP602]

Problem

Current chat message storage uses inefficient array mutations that create memory pressure and performance issues:

• [...messages, newMessage] copies entire message arrays on every new message
• For 600 messages: 600+ object copies per message addition
• Creates garbage collection pressure in busy chats (10+ msg/sec)
• Memory can grow to 4.5GB+ during extended use

Current Implementation Issues

1. Array mutation: Line 1510 in ChatProvider.jsx creates new arrays constantly
2. Sorting overhead: O(n log n) sort on every message when mixing optimistic/confirmed
3. Multiple array copies: Trimming creates additional array copies via slice()
4. Memory fragmentation: Scattered allocations instead of fixed buffers

Proposed Solution

Implement a circular buffer architecture:

class CircularBuffer {
  constructor(size) {
    this.buffer = new Array(size);
    this.size = size;
    this.head = 0;
    this.count = 0;
  }
  
  add(message) {
    this.buffer[this.head] = message;           // O(1)
    this.head = (this.head + 1) % this.size;   // O(1)
    this.count = Math.min(this.count + 1, this.size);
  }
}

Benefits

• O(1) insertion vs current O(n) array copy
• Fixed memory footprint - no growing/shrinking arrays
• Eliminates garbage collection pressure
• Better cache locality for message access
• Scales to higher message limits without performance degradation

Files to Modify

• src/renderer/src/providers/ChatProvider.jsx - Replace array-based storage
• Add circular buffer utility class
• Update message access patterns for rendering

Acceptance Criteria

• Messages stored in fixed-size circular buffers
• O(1) message insertion performance
• Memory usage remains stable during extended use
• All existing functionality preserved (paused mode, trimming, etc.)
• Performance improvement measurable in busy chats

This optimization will resolve the fundamental architecture issue causing memory leaks and enable higher message limits without performance degradation.