fix: route clipboard writes through Electron IPC to prevent NSPasteboard contention

[AmethystLiang]

Summary

• Routes all navigator.clipboard.writeText() calls through Electron's native clipboard module via IPC (window.api.ui.writeClipboardText())
• Adds clipboard:writeText IPC handler in the main process alongside the existing clipboard:readText
• Denies clipboard-read and clipboard-sanitized-write permissions to the renderer to prevent Chromium from autonomously polling NSPasteboard
• Eliminates clipboard contention that causes intermittent failures for concurrent CLI tools (e.g. Codex)

Test plan

• Verify copy path actions work in editor tab context menu, line gutter context menu, and search results
• Verify terminal copy (selection) still works
• Verify terminal paste still works
• Verify clipboard operations don't interfere with CLI tools running in terminal

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