fix: burn down frontend eslint errors; make CI lint blocking

Takes frontend/npm run lint from 23 errors / 40 warnings to 0 errors /
40 warnings, then removes continue-on-error from the CI lint step so it
gates merges.

Errors fixed outright:
- @typescript-eslint/no-explicit-any (5, ChatView.tsx): dropped
  redundant (msg as any) casts - Message already declares files,
  workflow, and error with the exact shapes the props expect.
- react/no-unescaped-entities (1, support/page.tsx): "We'll" ->
  "We'll".

Targeted disables (17, all react-hooks/set-state-in-effect): every site
is an intentional effect-driven state pattern - SSR/hydration mount
gates (Modal, useSelectedModel), reset-on-prop/identity-change
(CaseLawPanel x3, ChatView chat switch, CitationQuotesHeader,
ChatHistoryContext logout, useFetchDocxBytes), sync fast paths of async
fetch/check effects (CaseLawPanel, MfaLoginGate x2), DOM-measured state
(ChatView scroll button, message-visibility restore), and timed UI
latches (PreResponseWrapper, TRChatPanel, AskInputPopup auto-submit).
Rewriting any of them would change runtime behavior, so each carries a
// eslint-disable-next-line with a one-line reason instead of a fix or
a repo-wide rule change.

CI: lint step is now blocking; comments updated to say the backlog is
at zero.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>