feat(tui): add Project Files viewer/editor with vi mode support

[ryanwyler]

Summary

Adds a file browser and viewer/editor panel to the TUI for viewing and editing project files directly within OpenCode.

Activate vi prompt in command menu: Use vi basic prompt editor / Use standard prompt editor Activate vi editor in command menu: Use vi basic project file editor / Use standard project file editor

Depends on: #7016 (vi basic mode extension)

Features

• Project Files Browser - File tree in sidebar showing project files with expand/collapse
• File Viewer Panel - View files with syntax highlighting, resizable panel with drag divider
• File Editor - Edit files directly in the TUI with save (ctrl+s) and cancel (ctrl+c)
• Vi Mode Integration - Optional vi basic mode for editing (toggle with ctrl+/)
  • Status bar showing INSERT/NORMAL state and command buffer
  • Search: / forward, ? backward, n/N next/prev match
  • Search bar UI displays when typing search pattern
• Focus Management - Proper keyboard focus between editor and prompt
  • Click to refocus editor panel
  • Keys only captured when editor textarea is focused

Configuration

• editor_mode KV setting: "standard" or "vi" (default: standard)
• Toggle via ctrl+/ in edit mode or command menu

Files Changed

File	Changes
routes/session/project-files.tsx	File tree component
routes/session/content-panel.tsx	Panel wrapper with resize divider
routes/session/file-viewer.tsx	View/edit with vi mode integration
routes/session/sidebar.tsx	ProjectFiles integration
routes/session/index.tsx	ContentPanel + editor mode toggle