feat(ideas-foundation): scaffold Ideas app navigation and domain model

[ibourgeois]

Goal

Create the first domain foundation for the Ideas app inside the shared product shell.

Scope

• Add the Ideas toolbar entry with a lightbulb icon
• Create the initial idea model and storage design
• Enforce private ownership rules for draft ideas
• Add context navigation for the Ideas app

Out of Scope

• AI-assisted drafting
• Proposal conversion into projects
• Team review workflow

Acceptance Criteria

• The Ideas app appears in the main toolbar
• The domain model supports private idea ownership and future proposal flow
• Authenticated users can reach the Ideas area and only access their own private drafts

Dependencies

• Depends on feat(shell): create application shell and navigation architecture #4, feat(ui): create the first branded Blade component set #6, and feat(users): split user name fields and add computed attributes #8

Test Plan

• Add model and feature tests for ownership and access rules
• Manually verify the Ideas app appears and navigates correctly