feat(desktop): add Customize gallery and restyle Settings

[wangtsiao]

Summary

• Add an in-app Customize gallery plus MCP, Skills, and Rules settings, including plan-mode chat support from the earlier commit on this branch.
• Restyle Settings to match the new-chat homepage: large light titles, composer-like cards, and left-nav rows that use the same treatment as New chat / Search / Automations / Customize.
• Brand the Windows Electron taskbar icon in development, open ~/.devo/config.toml from Customize, list MCP tools without starting disabled servers, and permanently delete a folder's sessions when it is removed from the sidebar.

Test plan

• Open Customize from the sidebar and switch among Plugins, MCPs, Skills, and Rules.
• Open Settings and confirm titles, cards, and left nav match the home/sidebar chrome.
• On Windows, fully quit the client, run bun run brand:electron-dev, then bun run dev and confirm the taskbar icon is Devo rather than Electron.
• In Customize → MCPs, use Add and confirm ~/.devo/config.toml opens in the preferred editor.
• In Settings → MCP, list servers, expand tools, and toggle enablement without launching disabled servers.
• Remove a folder from the sidebar and confirm its sessions are deleted while the directory on disk remains.

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