fix(app): preserve provider dialog backdrop

[opencode-agent]

Summary

• keep provider selection, connection, and custom-provider steps inside one mounted dialog shell
• keep the modal dimensions stable while moving between provider steps so the home screen does not flash through
• preserve multi-method provider back navigation

Testing

• bun typecheck (packages/app)
• Prettier check on changed files

Evidence

• Before/after video
• Before: b44bc0ad0740afe80beae0a1981c2c4d96f67173
• After: 8cb65f96364305972157a8c32b928a1247a3a2eb
• 1280×800, light theme, 100% zoom, mocked backend data

Source discussion: https://discord.com/channels/1391832426048651334/1523038759623594135