Build Size Optimzation

[thatsjet]

Replace MDEditor with lightweight native textarea editor

Summary

• Remove the @uiw/react-md-editor third-party dependency and replace it with a native <textarea> and custom markdown formatting toolbar, significantly reducing bundle size and eliminating DOM hacking workarounds
• Lazy-load LLM provider modules via dynamic import() in the provider registry to improve startup performance
• Add file editor with line numbers, markdown toolbar, word wrap support, system prompt relocation, and keyboard shortcut fixes (from prior commits on this branch)

Changes

File Editor rewrite (src/renderer/components/FileEditor.tsx, src/renderer/styles/FileEditor.css)

• Replaced @uiw/react-md-editor with a plain <textarea> and a custom toolbar with bold, italic, strikethrough, link, quote, code, code block, bullet list, numbered list, and checklist actions
• Toolbar actions use document.execCommand('insertText') to preserve native undo/redo history
• Tab key inserts 2-space indentation
• Simplified scroll sync and line-height measurement — removed MutationObserver hacks that were needed to wait for MDEditor's internal DOM
• Removed the keydown capture-phase workaround that prevented MDEditor from swallowing system shortcuts (Cmd+Q, Cmd+C, etc.)

Provider lazy loading (src/main/providers/registry.ts, src/main/main.ts)

• getProvider() is now async with dynamic import() for Anthropic and OpenAI provider modules
• Both call sites updated to await getProvider(...)