Studio Code: accept image paste anywhere in the view

[sejas]

Related issues

• Fixes STU-1894

How AI was used in this PR

AI was used to trace the existing attachment pipeline, draft the change, and write the accompanying unit tests. The result was reviewed and verified manually.

Proposed Changes

Pasting an image into the Studio Code composer already works since #3925 — but only while the textarea itself is focused.

This PR makes both composers — the classic Studio Code tab (apps/studio) and the agentic UI session view (apps/ui) — accept image and file pastes globally anywhere in the conversation.

• Pasting an image (or file) while focus is anywhere in the Studio Code view attaches it and focuses the composer, ready to type.
• Pasting while the textarea is focused behaves exactly as before (no double-attach).
• Pastes inside open dialogs are left untouched, so modal inputs keep their normal paste behavior.
• Plain-text paste is unaffected everywhere.

The existing attachment limits (4 images, 5 MB per image, 12 MB total) apply unchanged.

Testing Instructions

1. Open a site → Studio Code tab, signed in to WordPress.com.
2. Take a screenshot to the clipboard (⌃⇧⌘4 on macOS).
3. Click somewhere in the conversation area (so the textarea is not focused), then press ⌘V → the image appears as an attachment chip and the textarea gains focus.
4. Press ⌘V again with the textarea focused → exactly one more chip appears (no duplicates).
5. Open the model-switch confirmation dialog (pick a model from another family) and paste → nothing is attached.
6. Send a message with a pasted image → the agent receives it.
7. Repeat steps 2–6 in the agentic UI (Feature flags → enable Agentic UI) session view.

copy-paste-iamge-agentic.mp4

copy-paste-image-classic.mp4

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[wpmobilebot]

📊 Performance Test Results

Comparing 1f8d0ef vs trunk

app-size

Metric	trunk	1f8d0ef	Diff	Change
App Size (Mac)	1352.96 MB	1352.96 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	1f8d0ef	Diff	Change
load	1097 ms	1067 ms	30 ms	⚪ 0.0%

site-startup

Metric	trunk	1f8d0ef	Diff	Change
siteCreation	6994 ms	7013 ms	+19 ms	⚪ 0.0%
siteStartup	2364 ms	2357 ms	7 ms	⚪ 0.0%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[bcotrim]

LGTM 👍