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/payments.ts

@@ -15,7 +15,7 @@ import {
   linkPaymentSchema,
   importPaymentsCsvSchema,
 } from '@/lib/validations'
-import type { Enums } from '@/types/database'
+import type { Enums, TablesInsert } from '@/types/database'
 
 type PaymentPlatform = Enums<'payment_platform'>
 
@@ -131,7 +131,7 @@ export async function importPaymentsCsv(rows: unknown) {
   // transaction_date. Invalid rows are skipped and counted, not silently
   // dropped without feedback.
   const rowSchema = importPaymentsCsvSchema.element
-  const inserts: Array<Record<string, unknown>> = []
+  const inserts: Array<TablesInsert<'payments'>> = []
   let skipped = 0
   for (const raw of rows) {
     const r = rowSchema.safeParse(raw)