[FEATURE] Template browser and project-from-template flows

[aernw]

Objective

Expose templates and starter sessions as a real creation flow in the web app.

Scope

• template discovery and browsing UI
• project creation from a selected template or starter session
• clear distinctions between normal projects and reusable starting points

Out of Scope

• [Features explicitly not included]
• [Future enhancements to defer]

Acceptance Criteria

• templates are easy to browse and reuse
• creating a project from a template feels like a product flow, not a manual clone trick

Dependencies

Blocked by: (none)

Blocks: (none)

Success Metrics

• [How to measure success - optional]

Roadmap Context

Milestone: M6 - Productization & Coverage Expansion
Parent workstream(s): #212, #163