fix(p0): resolve active domain automatically when onboarding skipped

- New resolveActiveDomain() helper in apps/web/lib/auth.ts: returns the
  user's first existing domain (ordered by sort_order then created_at),
  or auto-creates a default 'Personal' domain if they have none.
- 7 affected API routes (agents, canvases, domains, habits, projects,
  search, tasks) now fall back to resolveActiveDomain when the request
  omits a domain param. This eliminates the UNDEFINED_VALUE on domain_id
  and 22P02 invalid uuid errors that broke 6+ UI flows.
- New POST /api/quick-capture route: switches on type=task|habit|note|project
  to create the right entity, after resolving the active domain. This is
  the API the dashboard quick-capture widget calls.
- packages/db/src/schema.ts: added ownerId: uuid('owner_id') to the
  domains table so each user owns their domains.
- drizzle/0004_add_owner_id_to_domains.sql: matching migration with
  column add + index.
- apps/web/__tests__/lib/auth.test.ts: unit tests for both branches of
  resolveActiveDomain (returns existing / creates Personal).
- apps/web/app/(dashboard)/canvas/page.tsx: removed hardcoded
  domain: 'personal' from the quick-create payload so server-side
  resolution can do its job.

This unblocks all of: /tasks, /habits, /projects, /search, /settings/agents,
/settings/domains, /canvas, and dashboard quick-capture.

drizzle/0004_add_owner_id_to_domains.sql

@@ -0,0 +1,3 @@
+ALTER TABLE "domains" ADD COLUMN "owner_id" uuid REFERENCES "public"."users"("id") ON DELETE cascade ON UPDATE no action;
+--> statement-breakpoint
+CREATE INDEX "domains_owner_id_idx" ON "domains" USING btree ("owner_id");