fix(desktop): read composer image preview locally in remote mode

[Kyzcreig]

Problem

When the desktop app runs against a remote gateway (thin client on one Mac driving a hermes dashboard on another), pasting an image into the composer pops "Image preview failed" / "file not found" every time.

Root cause

A pasted/clipboard/dropped/picked composer image is written to the client's local Electron userData/composer-images/ dir (saveImageBuffer / saveClipboardImage) — even in remote mode, because that's the only place the clipboard bytes exist. But the preview readback (attachImagePath) went through readDesktopFileDataUrl(), which in remote mode routes to the backend's GET /api/fs/read-data-url. The backend is a different machine and has no such file → 404 File not found → the "Image preview failed" toast.

Verified live: the pasted PNG exists on the client Mac's disk; the exact path does not exist on the backend Mac, and _fs_regular_file raises 404 "File not found" for it.

The attachment itself was unaffected — submit uploads the bytes via image.attach_bytes, which already reads the local path through the Electron bridge (readImageForRemoteAttach → window.hermesDesktop.readFileDataUrl). Only the thumbnail preview was broken.

Fix

Add readComposerImagePreview() which reads the preview from the local Electron bridge first, falling back to the remote-aware read only when the local bridge is unavailable or can't read the path (e.g. a path dragged from the remote file tree). This mirrors what the submit-time upload already does, and keeps local-mode behavior identical.

Tests

3 new cases in desktop-fs.test.ts:

• remote mode reads the local bridge and never touches backend FS REST (the regression guard)
• local mode still reads the local bridge
• remote-file-tree path falls back to the backend read when the local read throws

apps/desktop typecheck, eslint, and prettier all clean; the two touched test files are green (22 tests).

[greptile-apps]

Greptile Summary

This PR fixes a "Image preview failed" regression in remote (thin-client) mode by introducing readComposerImagePreview(), which reads composer images from the local Electron bridge first and falls back to the remote-aware readDesktopFileDataUrl() only when the local read throws.

• desktop-fs.ts: Adds readComposerImagePreview() — tries window.hermesDesktop.readFileDataUrl (local bridge) first; swallows errors and falls through to the backend FS REST only for paths that don't exist on the client (e.g. dragged from the remote file tree).
• use-composer-actions.ts: Replaces the single readDesktopFileDataUrl call in attachImagePath with the new readComposerImagePreview.
• desktop-fs.test.ts: Adds 3 targeted regression tests covering remote-local, local-local, and the ENOENT fallback case.

Confidence Score: 5/5

Safe to merge. The change is a narrow, targeted fix with no impact on submit-time attachment behaviour.

The fix is minimal and well-scoped: one new function in desktop-fs.ts, one call-site update, and three regression tests that specifically guard the broken code path. The local-first read correctly mirrors what the submit-time attachment path already did. All existing tests continue to pass, and the fallback to the remote-aware read preserves functionality for paths that originate from the remote file tree. No shared state, no side-effects outside the preview thumbnail path.

No files require special attention.

Important Files Changed

Filename	Overview
apps/desktop/src/lib/desktop-fs.ts	Adds readComposerImagePreview() with local-first read and silent error fallback. Logic is sound; one subtle edge: a falsy (empty-string) return from the local bridge also triggers the fallback, which in remote mode issues an unnecessary backend request.
apps/desktop/src/app/chat/hooks/use-composer-actions.ts	Single call-site swap from readDesktopFileDataUrl to readComposerImagePreview; existing try/catch error-handling is preserved and still shows the Image preview failed toast on failure.
apps/desktop/src/lib/desktop-fs.test.ts	Adds a focused describe block with 3 new tests; all key paths (remote-local, local-local, ENOENT fallback) are covered. The no-bridge case and empty-string-return case are not explicitly tested but are not regression risks.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[attachImagePath called\nwith filePath] --> B[readComposerImagePreview]
    B --> C{window.hermesDesktop\n.readFileDataUrl available?}
    C -- No --> F[readDesktopFileDataUrl]
    C -- Yes --> D[localReader path]
    D -- Returns non-empty string --> E[Return local data URL\n✓ Preview shown]
    D -- Returns empty string --> F
    D -- Throws e.g. ENOENT --> F
    F --> G{Remote mode?}
    G -- No --> H[bridge.readFileDataUrl path\nLocal Electron IPC]
    G -- Yes --> I[GET /api/fs/read-data-url\nBackend REST]
    H --> J[Return data URL]
    I --> J
    H -- Throws --> K[Error propagates to caller]
    I -- 404 Not Found --> K
    K --> L[notifyError\nImage preview failed toast]
    J --> M[addComposerAttachment\nwith previewUrl]

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flowchart TD
    A[attachImagePath called\nwith filePath] --> B[readComposerImagePreview]
    B --> C{window.hermesDesktop\n.readFileDataUrl available?}
    C -- No --> F[readDesktopFileDataUrl]
    C -- Yes --> D[localReader path]
    D -- Returns non-empty string --> E[Return local data URL\n✓ Preview shown]
    D -- Returns empty string --> F
    D -- Throws e.g. ENOENT --> F
    F --> G{Remote mode?}
    G -- No --> H[bridge.readFileDataUrl path\nLocal Electron IPC]
    G -- Yes --> I[GET /api/fs/read-data-url\nBackend REST]
    H --> J[Return data URL]
    I --> J
    H -- Throws --> K[Error propagates to caller]
    I -- 404 Not Found --> K
    K --> L[notifyError\nImage preview failed toast]
    J --> M[addComposerAttachment\nwith previewUrl]

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