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>

app/(app)/forms/[id]/results/page.tsx

@@ -30,7 +30,16 @@ export default async function FormResultsPage({ params }: { params: Promise<{ id
       formId={form.id}
       title={form.title}
       fields={parseFormSchema(form.schema)}
-      submissions={submissions ?? []}
+      submissions={
+        (submissions ?? []) as {
+          id: string
+          member_id: string | null
+          submitter_email: string | null
+          answers: Record<string, unknown> | null
+          form_version: number
+          created_at: string
+        }[]
+      }
     />
   )
 }

app/(app)/forms/[id]/results/results-client.tsx

@@ -51,6 +51,7 @@ export function ResultsClient({
       toast.error(res.error || 'Export failed')
       return
     }
+    if (!res.data) return
     const { filename, csv } = res.data
     const blob = new Blob([csv], { type: 'text/csv;charset=utf-8' })
     const url = URL.createObjectURL(blob)