types: clear remaining tsc errors across 41 files (76→14, behavior-preserving)

Per-file type-only fixes surfaced after the Supabase client was correctly
typed. All fixes preserve runtime behavior; no any/ts-ignore/ts-expect-error.

- lib/supabase/client.ts: annotate the browser client as SupabaseClient<Database>
  (same ssr/js drift as server.ts; fixes it centrally for all callers).
- Supabase embedded-select joins whose FK metadata is missing from the
  generated types are bridged with `as unknown as <target>` + a TODO to remove
  after regenerating types/database.ts.
- setState optimistic-update variance and raw <select> enum values cast to the
  precise element/union type; .insert payload fields (Json, enums) given the
  precise column type via satisfies/field casts.
- Added narrowing guards where a value was legitimately nullable (comms channel,
  public form page) without changing the intended path.

The 14 remaining errors are confined to the four files fixed in Phase B (real
bugs + the tautological actions.test rewrite). 639 tests still green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/actions/onboarding.ts

@@ -3,6 +3,7 @@
 import { revalidatePath } from 'next/cache'
 import { createClient } from '@/lib/supabase/server'
 import { createAdminClient } from '@/lib/supabase/admin'
+import type { Json } from '@/types/database'
 import { requireMember, requireMemberWithRole, parseInput } from '@/lib/auth-helpers'
 import {
   createOnboardingStepSchema,
@@ -45,7 +46,7 @@ export async function createOnboardingStep(input: {
       step_type: v.data.step_type,
       title: v.data.title,
       body: v.data.body ?? null,
-      config: v.data.config ?? {},
+      config: (v.data.config ?? {}) as Json,
       is_enabled: v.data.is_enabled ?? true,
       is_required: v.data.is_required ?? false,
       is_system: false,