chat: support video attachments

[octo-patch]

Reason: MiniMax-M3 supports video input, but chat attachments did not validate or encode video files.

• Accept MP4, AVI, MOV, and MKV uploads with the documented video and request size limits.
• Normalize browser MOV uploads and encode video attachments as base64 video content blocks.
• Allow video selection in the chat UI and apply the matching client-side size limit.
• Add regression coverage for video validation, MIME mapping, and message construction.

Checks:

• bun test src/chat/__tests__/upload.test.ts src/chat/__tests__/message-builder.test.ts
• bun test
• bun run lint
• bun run typecheck
• cd chat-ui && bun run test
• cd chat-ui && bun run typecheck
• cd chat-ui && bun run build