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) <[email protected]>

__tests__/markdown-anchors.test.ts

@@ -1,4 +1,5 @@
 import { describe, it, expect } from 'vitest'
+import type { ReactNode } from 'react'
 import {
   headingText,
   slugifyHeading,
@@ -13,7 +14,7 @@ describe('headingText', () => {
   })
   it('flattens arrays and element children', () => {
     expect(headingText(['Get ', 'started'])).toBe('Get started')
-    expect(headingText({ props: { children: ['Bold ', { props: { children: 'bit' } }] } })).toBe('Bold bit')
+    expect(headingText({ props: { children: ['Bold ', { props: { children: 'bit' } }] } } as ReactNode)).toBe('Bold bit')
   })
   it('ignores null/boolean', () => {
     expect(headingText(null)).toBe('')