feat(images): open image files as pictures, not binary refusals

[letrquan]

What

Opening an image file in Chapter used to hit a dead end — the changes list, the Ctrl+P palette and the commit panel all routed images into a diff or code view, which can only answer with a "Binary file" notice. Images rendered solely as assets inside markdown previews.

This adds a first-class image view: clicking an image file (png/jpg/jpeg/gif/webp/avif/bmp/ico/svg) now opens it as a picture in its own tab.

How

Frontend (Chapter.Web)

• New image view mode alongside diff/code/preview. The image is fetched as a data URI through the existing getAsset bridge endpoint and rendered centred in a new #image-host pane, on a checkerboard background so PNG/SVG transparency reads as transparency rather than as the theme colour.
• Images open straight into image view from every entry point (changes list default, palette navigation, markdown links, commit panel). The Diff/Code segmented control hides for image tabs — nothing to switch to — and the Ctrl+D toggle stands down. The hunk bar is dismissed when an image replaces a split diff, so it cannot stage against a file no longer on screen.
• The extension predicate mirrors the backend allow-list, with comments pointing at each other.

Backend (Chapter.Core)

• getAsset is now scope-aware, mirroring getFileContent: Branch/Uncommitted read from disk; Committed/LastCommit resolve where the comparison ends and serve the image as that revision has it — an edited picture can no longer leak into a review of older work.
• New DiffService.GetBytesAtAsync returns raw bytes at a revision (git show rev:path, null when absent) for content that must not pass through a text decode — GetContentAtAsync classifies images as binary and would hand back an empty string.
• Every failure is still reported as an AssetPayload reason ("not found", "too large to preview", …) rather than thrown, so the UI can show an honest refusal; the one new throw path (git failure on committed scopes) is caught by the existing per-call error handling, matching getFileContent behaviour.

Verification

• dotnet test: 441 passed (2 asset tests updated for the instance API; new Opened_images_follow_the_scope_not_the_working_tree proves a Committed-scope image serves the committed bytes, not the working tree).
• tsc --noEmit clean; npm run build clean.
• Verified in the running app: image files open as pictures from the changes list, the mode switch hides for them, and text files are unaffected.

Notes for reviewers

• Mode (main.ts) and ViewMode (editor.ts) are parallel unions; drift is caught at compile time at the showMode/revealPosition call sites.
• Known cosmetic gap, shared with the pre-existing "Binary file" notice path: a refusal notice does not clear a stale banner from the previous text tab (banner is z-index 30, above the notice).

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Project	Deployment	Actions	Updated (UTC)
chapter	Error		Aug 25, 2026 4:33am