[Model] Add missing relationships, query scopes, and factory states

[ibourgeois]

Problems

• User model is missing a projects() HasMany relationship despite projects.user_id FK existing
• Idea model has no shared()/draft() scopes despite a DB index on (user_id, shared_at) designed to support this pattern
• Project model has no proposed() scope despite a DB index on (idea_id, status)
• UserFactory doesn't include the theme_preferences column (added via migration)
• IdeaFactory has no shared() state, making it hard to test any sharing logic

Required Changes

• User: add public function projects(): HasMany returning $this->hasMany(Project::class)
• Idea: add scopeShared() (whereNotNull('shared_at')) and scopeDraft() (whereNull('shared_at'))
• Project: add scopeProposed() (where('status', 'proposed'))
• UserFactory: include theme_preferences with a default value of ['mode' => 'system']
• IdeaFactory: add shared() state that sets shared_at to now()

Files

• app/Models/User.php
• app/Models/Idea.php
• app/Models/Project.php
• database/factories/UserFactory.php
• database/factories/IdeaFactory.php