feat(ideas-editor): implement private idea draft CRUD

[ibourgeois]

Goal

Let users capture and iterate on private ideas over time before proposing them.

Scope

• Quick capture flow
• Create, edit, and revisit private drafts
• Save partial work and return later

Out of Scope

• AI-assisted idea iteration
• Team review and approval workflow
• Proposal-to-project conversion

Acceptance Criteria

• Users can create and edit private ideas
• Users can return to unfinished drafts later
• Users cannot access another user's private drafts

Dependencies

• Depends on feat(ideas-foundation): scaffold Ideas app navigation and domain model #21

Test Plan

• Add CRUD feature tests and ownership checks
• Manually verify quick capture and revisit flows