feat(projects): make project creation channel-first

[thomaspblock]

Summary

• create explicit NIP-MP projects with a home channel and default repository
• preserve standalone repository folding, project deletion, and deterministic repository selection
• restore Template, Team, visibility, and agent settings in the project creation flow

This is Part 2 of the channel-first Projects stack, following #6590. It is independently based on main; Part 3 adds the project-home channel surface.

Testing

• focused project collection, creation, channel, and model tests: 38/38 passed
• Desktop unit suite: 5,415/5,415 passed
• TypeScript, Biome, and differential file-size checks passed
• full pre-push gate passed

Post-Deploy Monitoring & Validation

• create listed and unlisted projects with and without templates in the first staging Desktop session
• healthy signals: one home channel, one default repository, stable project coordinates, and no duplicate legacy card
• failure signals: partial project creation, duplicate projects, missing default repository, or stale sidebar entries; mitigate by reverting this PR