feat: Add session templates with pre-filled cards

[camilocbarrera]

Summary

Add a template system that allows users to start sessions with pre-configured cards for common collaborative activities like retrospectives, brainstorming, flowcharts, and more.

Problem

Users start with a blank board every time. For structured activities like sprint retrospectives or user journey mapping, they need to manually create category cards and set up the board layout—repetitive and time-consuming.

Proposed Templates

Template	Pre-filled Cards
Sprint Retrospective	What went well, What could improve, Action items
Brainstorm	Goal/Problem, Ideas zone, Top picks
User Journey Map	Awareness, Consideration, Purchase, Delivery, Loyalty
Pros & Cons	Pros column, Cons column
SWOT Analysis	Strengths, Weaknesses, Opportunities, Threats
Kanban Board	To Do, In Progress, Done

Technical Approach

1. Add templates table with id, name, description, cards (jsonb)
2. Add createSessionFromTemplate() server action
3. Create TemplateSelector component on home page
4. Seed built-in templates on first deploy

Acceptance Criteria

• At least 5 built-in templates available
• Template cards positioned logically on canvas
• Users can modify/delete template cards after creation
• Works on mobile
• No loading state needed (instant load)