fix: remove unsupported HTML upload option from admin document picker

[sandeep-2536]

What was broken

The admin document-upload UI in AdminContextSources.tsx advertised HTML
(.html/.htm) as an accepted file type — in the file picker's accept
attribute, the upload subtitle text, and the error message — but the
backend has never supported it. The multer filter and controller validation
in admin-documents.routes.ts / adminDocuments.controller.ts only allow
PDF/TXT/Markdown/CSV, so any HTML upload was silently rejected with a 400.

Git blame confirms this wasn't accidental: the original commit's comment
explicitly says HTML support "will still be rejected by the backend until
ALLOWED_MIME... is extended (tracked as a TODO)" — planned follow-up work
that was never completed.

Fix

Removed HTML/HTM from the upload-specific UI only:

• File input accept attribute (.pdf,.txt,.md,.csv,.html,.htm → .pdf,.txt,.md,.csv)
• Upload card subtitle text
• Upload error message
• The now-resolved TODO comment above the input

The separate "Paste text or HTML" feature (pasting raw text/HTML directly,
not file upload) is untouched — that's a different feature path with its
own handling and was intentionally left as-is.

Why this approach

Considered two options: (A) remove HTML from the frontend to match backend
reality, or (B) implement real backend HTML parsing/extraction. Went with
(A) because (B) would require new parsing logic, security hardening
(sanitization, since HTML is untrusted input), and test coverage — a
reasonable future feature, but not a fit for a bug-fix PR. (A) directly
closes the mismatch with no functional risk.

How this was found

Found via manual code review, not from an assigned GitHub issue.

Testing

• Updated AdminContextSources.test.tsx to assert the file input's
  accept value is now .pdf,.txt,.md,.csv
• Manually tested: .html/.htm no longer selectable in the file picker,
  subtitle/error text read correctly, PDF/TXT upload still works unchanged
• pnpm exec tsc --noEmit — clean, no errors
• pnpm test — AdminContextSources.test.tsx passes (9/9). 3 unrelated
  test suites (api.test.ts, PremiumTee.test.ts,
  GoldenTicketDetailPage.test.tsx) fail on main as well, due to a
  pre-existing tooling error (jsTokens is not a function) unrelated to
  this change.