improvement(web): split Import/Export into tabs with a source picker

[croffasia]

What

The Import/Export settings page is split into an Import tab and an Export tab. The Import tab starts with a source picker: Plane is selectable, Linear, Jira and Trello are listed disabled with a "Soon" mark. The Plane API token field is a masked text input instead of a password input.

Why

Import and export were one long page with no separation, and there was no place to add further import sources. A password input made Chrome offer saved logins for the Workspace field above it and ignore autocomplete="off"; a masked text field does not trigger that.

How to test

1. Open Project settings → Import/Export.
2. The page shows Import and Export tabs; Export is hidden without import_export: create.
3. On Import, the Source row shows Plane selected and Linear / Jira / Trello disabled.
4. Below it the Plane connect form, source project and jobs list work as before.
5. Focus the Workspace field: the browser offers no saved logins. The API token field still shows dots.
6. On Export, the "Export project data" button downloads the JSON.

Checklist

• bun run typecheck passes
• bun run lint and bun run format:check pass
• Tests added or updated for the changed behaviour
• Database schema changed: migration generated with bun run db:generate and committed
• New environment variables documented in .env.example
• Docs updated (README.md or the relevant AGENTS.md)

Screenshots

Import tab: line tabs Import | Export, then a Source row with Plane active and the other sources dimmed, then the Plane connect form and jobs list.