feat: email template library + /templates page + Browse picker (HIR-75)

[jaredzwick]

Summary

Adds the first 'move the needle' README TODO: a starter email template library.

• src/lib/templates/catalog.ts — 6 seeded templates (saas_onboarding, agency_outreach, recruiter_outbound, b2b_warm_intro, founder_to_founder, re_engagement_silent_3day) using the existing {{variable}} placeholder syntax from the campaigns API (src/app/api/campaigns/route.ts).
• /templates (Next.js app router) — grid of template cards with preview + 'Use this template' that hands off to the sequence form.
• /dashboard/campaigns/new — single-step sequence-creation form with a 'Browse templates' button that opens the same picker as a modal, and ?templateId= prefill from the templates page.
• tests/int/templates.int.spec.ts — 10 unit tests (vitest) verifying:
  • all 6 required templates are seeded with unique IDs
  • every {{placeholder}} in subject/body is declared in variables[]
  • every declared variable is actually used in the template text
  • extractPlaceholders and getTemplateById behave correctly

Notes for reviewers

• I went with {{first_name}} (existing campaigns API syntax) rather than the {first_name} shape mentioned in the issue body — per the engineer's note to match the existing templating system.
• Repo had no UI for sequence creation yet (only the /api/campaigns endpoint), so I added a minimal one at /dashboard/campaigns/new. That gives the modal somewhere to live without introducing a new abstraction.
• Out of scope (follow-ups): template editing UI, user-saved custom templates, AI generation.

Test plan

• pnpm exec vitest run --config ./vitest.config.mts tests/int/templates.int.spec.ts — 10/10 pass
• pnpm exec next lint clean on all new files
• pnpm exec tsc --noEmit — no new type errors introduced (pre-existing errors in libs/db, src/app/api/campaigns/route.ts, etc. are unchanged)
• Visual QA on /templates and /dashboard/campaigns/new (auth-gated, deferred to local run)

Definition of done

• PR open against pypesdev/coldflow main branch
• All 6 templates render in /templates page (rendered via TemplatePicker)
• Picker prefills sequence-creation form correctly (URL param + modal handler both call applyTemplate)
• Catalog test green in CI
• README's 'Move the needle TO-DO' templates checkbox ticked

Closes HIR-75. Parent: HIR-74.