fix(app): preview image attachments delivered by path - #57249

Problem

The composer only shows a thumbnail for images whose bytes are in the draft store. An image attached as a path attachment shows a text card with the filename and PNG instead. That happens when the image is a file the server reads by path rather than inline: it is too large to send inline, the selected model has no image input, or the composer had no resolved model when the file was added.

Fix

• Path attachments with an image/* MIME type now render the same thumbnail as inline images. The bytes are read through the server's fs/read (readLocalImage, the same route Markdown images use), so both local source paths and paths uploaded to a remote server work.
• Clicking the thumbnail opens the existing image preview for both kinds of attachment.
• If the file can't be read (for example, it was deleted), the chip falls back to the existing filename card instead of showing an empty box.
• Object URLs are cached per path in the composer and revoked when it unmounts. Pending reads are aborted.
• Thumbnail loading reads resource state instead of suspending, which keeps the existing focus-preserving behavior without the local Suspense.
• readLocalImage now accepts Windows drive paths that use backslashes (C:\Users\...\image.png). Previously it rejected them as a URL scheme, so a file reference on Windows couldn't load.

Verification

• bun run typecheck in packages/app.
• image.test.ts: a new backslash drive-path case, which fails before the fix.
• The composer mixed-attachments story now includes a path image attachment, and its existing component test asserts the thumbnail renders. That assertion fails before the editor change. I checked it against Storybook in Chromium; the full Playwright component harness couldn't launch a browser on my host.
• Not run on a real Windows desktop: the Windows dev box was unreachable.

Requested by: @Hona (Hona via Slack)