Migrate to Tauri desktop app with terminal, file watcher, and table support

[jalexray]

Summary

• Tauri migration: Replace Flask+Vite two-process architecture with a single Tauri macOS app. React frontend runs in a native WebView, Rust backend handles file I/O, git operations, and config via IPC commands.
• Embedded terminal: Toggleable terminal panel using portable-pty (Rust) and xterm.js, spawning the user's login shell in the repo directory.
• File watching: Rust notify crate watches the repo for .md file changes, emits Tauri events to auto-refresh the editor and file tree. Debounced and filtered to avoid noise.
• Table support: Add TipTap table extensions so markdown tables render as formatted, resizable tables in the WYSIWYG editor.
• Project docs: Initialize brain-implant for knowledge management, add design system prompt, update history and sample docs.

Test plan

• cargo tauri dev launches the app with sidebar, editor, git panel, and terminal
• Open a markdown file with a table — verify it renders as a formatted table
• Edit a .md file externally (e.g., from terminal) — verify the editor auto-refreshes
• Create/delete a .md file externally — verify the sidebar file tree updates
• Save a file via Cmd+S — verify no duplicate refresh or content flicker
• Stage, commit, and push workflow works in the git panel
• Terminal spawns shell, supports color, persists when panel is hidden

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