feat(cli): paste base64 / data URL images, drag image files, with [Image #N] placeholders

[BingqingLyu]

Summary

Adds three new ways to attach images to the prompt and unifies them with the existing Cmd+V path under a single [Image #N] placeholder UX.

Closes QwenLM#3518

What's new

1. Data URL text paste — paste data:image/<type>;base64,<payload> (devtools "Copy as data URL", chat messages containing embedded images, etc.).
2. Raw base64 paste — accepted only when the decoded prefix matches a known image magic (PNG / JPEG / GIF / WebP / BMP / TIFF). JWTs, hashes, and base64-looking text without a valid image magic are rejected so normal paste flows aren't hijacked.
3. Drag an image file into the terminal — works on terminals that wrap the drop in bracketed paste AND on terminals (macOS Terminal.app is the big one) that synthesize the drop as a rapid burst of individual keystrokes, via a simple < 10 ms keystroke-gap heuristic + a 150 ms debounced scan of buffer.text's trailing token.

Unified UX

All four sources — Cmd+V binary clipboard (existing), data URL, raw base64, drag-drop — converge on:

• Inline [Image #N] placeholder inserted at the cursor.
• Matching chip in the attachment row.
• Monotonic counter per input, resets on submit.
• [Image #N] → @<relative path> substitution at submit time so the model sees each image exactly where the user placed it.

> please compare [Image #1] against [Image #2]
Attachments: [Image #1] [Image #2]

Attachments added without a placeholder (legacy code paths) keep rendering as [filename], so other callers are unchanged.

Code

• clipboardUtils.ts:
  • tryDecodeBase64Image(text) — data URL + raw base64, magic-byte sniff.
  • saveDecodedImage(buf, ext, dir) — persists decoded bytes to the shared clipboard temp dir.
  • detectDraggedImagePath(text) — validates a token is an existing local image file; supports single/double-quoted paths and escaped spaces.
  • Existing clipboardHasImage / saveClipboardImage / cleanupOldClipboardImages unchanged.
• InputPrompt.tsx:
  • Paste branch order: pasteImage → data URL / raw base64 → drag-drop paste path → large paste placeholder → small paste.
  • Keystroke-burst detection fallback for drag-drops that arrive as typing (see constants DRAG_BURST_MAX_INTERVAL_MS = 10, DRAG_MIN_BURST_CHARS = 4, DRAG_CHECK_DEBOUNCE_MS = 150).
  • [Image #N] allocator + [Image #N] → @path substitution in handleSubmitAndClear.

Test plan

• npm test --workspace packages/cli -- clipboardUtils InputPrompt — 143 pass, 1 Windows-skip, 0 fail
  • tryDecodeBase64Image: data URL, declared-vs-sniffed MIME (magic wins), raw base64 PNG, raw base64 JPEG, rejects text / JWT-like / empty / short / non-base64 data URL.
  • saveDecodedImage round-trips bytes to disk.
  • detectDraggedImagePath: existing file, quoted path, escaped-space path, non-image extension, missing file, directory, empty input, multi-line.
  • InputPrompt > base64 / data URL paste: single paste, sequential [Image #1]/[Image #2], fall-through to large-paste placeholder for non-images.
  • InputPrompt > drag-and-drop image paste: image path → [Image #1], non-image path → regular buffer handling.
• npm run build --workspace packages/cli passes.
• Manual on darwin-arm64, Node 22.17: data URL paste, screenshot Cmd+V, Finder drag onto Terminal.app, Finder drag onto iTerm, non-image drag, empty submit.

Backwards compatibility

• All public signatures unchanged (clipboardHasImage, saveClipboardImage, cleanupOldClipboardImages).
• Attachment chip rendering falls back to [filename] when no placeholder is set.
• No change to multipart message assembly or to @path mentions outside the image paste flow.
• No change to dependencies.

Notes

• The burst heuristic thresholds are tuned to only trigger on mechanical input storms; human typing won't fire the detection.
• The raw-base64 branch is conservative (decoded prefix must match a known image magic) so common base64-looking pastes — auth tokens, hashes, arbitrary data — are not turned into images.
• Not in scope: the pre-existing @teddyzhu/clipboard binding issues on some platforms (Clipboard image paste (Cmd+V) silently fails on macOS — two root causes QwenLM/qwen-code#3517). This PR leaves that path untouched.