perf(web): optimize document upload validation with static sets, MIME checks, and 50MB size guard

[muskanai0409-cell]

Fixes #1559

What changed

Refactors isAcceptedFile() in apps/web/components/add-document/file.tsx per the issue:

• Static sets hoisted to module scope — ALLOWED_EXTENSIONS and ALLOWED_MIME_TYPES are now created once at module load instead of re-instantiating a Set on every call, eliminating redundant heap allocations during batch drag-and-drop.
• 50MB fail-fast size guard — isAcceptedFile() now rejects files over MAX_FILE_SIZE_BYTES (50MB), matching the limit already enforced by chat attachments (CHAT_ATTACHMENT_MAX_BYTES in apps/web/components/chat/attachments.ts) and the backend workers. Oversized files never start uploading, and addFiles() shows a dedicated toast ("One file exceeds the 50 MB limit") separate from the unsupported-type toast, so users get accurate feedback.
• Direct O(1) MIME verification — the file's MIME type is checked against ALLOWED_MIME_TYPES first (application/pdf, Word/Excel Office types, text/csv, text/plain, text/markdown, application/json, text/html) with image/* prefix support, so extensionless mobile uploads (e.g. a blob with type: "application/pdf") are accepted. MIME parameters such as ;charset=utf-8 are stripped before the lookup. The extension check is kept as a fallback for files with no/unknown MIME type.
• application/pdf added to FILE_ACCEPT, aligning it with CHAT_ATTACHMENT_ACCEPT.

Behavior notes

• Previously accepted files with a supported extension are still accepted; the only newly rejected inputs are files >50MB, which the gateway rejected anyway.
• Newly accepted: extensionless files whose MIME type is a supported document type (previously only markdown/JSON/HTML/image MIME types passed).

Verification

• Unit-tested the validator with bun test (extension matrix, case-insensitivity, extensionless MIME acceptance, charset parameters, image/*, exact-50MB boundary, >50MB rejection, unsupported types) — 12 tests, all passing locally.
• bunx biome check on the changed file: clean.
• tsc --noEmit in apps/web: no new errors introduced (the 84 pre-existing errors on main are unchanged).

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