feat(chat): attach any file from the composer's button, not only images (#1101)

[agegr]

On a phone there is no drag and drop, so the only way to give the chat a PDF or a log was the attach button, which took images only (#1101). Approved scope: the button accepts any file; no staging area outside the project.

• components/ChatInput.tsx: new optional onAttachFiles(files). With it, the hidden file input has no accept (and no capture, so phones still offer the photo library and camera) and hands the whole selection over; without it, the input stays accept="image/*" as before.
• components/ChatWindow.tsx: onAttachFiles runs the picked files through the existing chat drop handler (feat(chat): drop files onto the chat to upload them and mention them #1094), so they behave exactly like a drop:
  • images attach to the prompt (same limits, compression, image-support warning);
  • other files upload into the working directory with conflict=skip and come back as @ mentions, with the same notices;
  • a mixed pick does both; with no working directory, images still attach and other files get the same warning as a drop.
• i18n: chat.attachImage → chat.attachFiles ("Attach files" / "附加文件" / "附加檔案"); the no-directory notice says "adding" instead of "dropping" since it covers both.
• docs/agents/files-and-access.md: one bullet.

Tests: components/ChatInput.test.mjs (accept/capture/label with and without the handler; the real handleFilePick routing), new components/ChatWindow.attach-files.test.mjs (real drop/upload path with a stubbed upload request: mixed pick, image-only pick, no working directory). tsc, eslint, npm test pass.

Not checked on a real phone. Per WebKit 267277, iOS converts HEIC to JPEG only when accept lists specific types that exclude HEIC, which image/* did not, so dropping accept should not change the format of picked photos.

Refs #1101

🤖 Generated with Claude Code