Fix diff EOF rendering, commit draft loss, LFS preview; polish DAG

[hewigovens]

Summary

Five fixes + polish bundled together from one debugging session:

• Diff engine (jj-diff) — Rust's .lines() silently drops the trailing newline, so an EOF-newline-only change produced an all-Equal diff and rendered as plain file content. Added no_eof_newline on DiffLine + whitespace_only_hidden on FileDiff. Post-process synthesizes a visible Remove+Add pair (with spans kept as Context so there's no spurious word-level highlight) and marks the last affected line per side when EOF differs alongside real changes.
• Diff renderer (NativeDiffView) — render ⊘↵ no newline at EOF in dim gutter color on lines carrying the flag. DiffSection shows a small orange banner when whitespaceOnlyHidden suppressed real changes.
• Commit box draft loss — CommitBox lives inside if shouldShowCommitBox, so switching to any other change unmounted it and destroyed @State draft. Hoisted to RepoViewModel.commitDraft via Binding. Also guarded seeding with draft.isEmpty so background workingCopyDescription refreshes no longer clobber in-progress typing.
• LFS pointer image preview — when LFS is uninstalled and a pointer file is committed under an image extension, extract_image_preview used to cache the ~130-byte pointer text as a bogus .png, leading to an infinite AsyncImage spinner. Now sniffs LFS pointer bytes first; caller emits the same <git lfs pointer ...> placeholder the text path already knows how to render.
• DAG node styling — new DAGNodeStyle helper. Trunk-bookmarked commits (main / master / trunk, with optional @origin / @upstream) render as diamonds; other bookmarked commits render as hollow accent-stroked circles. Rebase overlays now match the node shape.
• Bookmark copy button — extracted CopyIconButton (the icon + flash pattern that already lived inside CopyableRow) and dropped one next to each bookmark capsule in the Detail header.

Test plan

• `cargo test -p jj-diff` — 31 pass incl. 4 new tests: EOF-newline added/removed synthesizes a visible pair, marker on real changes, and `whitespace_only_hidden` flag for `ignore_whitespace`-suppressed diffs
• `just test` — all Rust crates green (79 tests)
• `just test-app` — all Swift unit tests green (22)
• `cargo clippy --workspace` + `swiftlint` clean
• Open a file with trailing-newline-only change: see `⊘↵ no newline at EOF` on the side that lacks it, line text is not word-highlighted red/green
• Type in commit box, select another change, select @ again: draft is preserved
• Open a repo with an uninstalled LFS pointer image: placeholder renders instead of infinite spinner
• DAG view with a `main` bookmark: trunk commit renders as a diamond; side bookmarks as outlined circles
• Detail view with bookmarks: copy icon flashes ✓ on click

[gemini-code-assist]

Code Review

This pull request enhances the diffing engine and UI by adding support for "no newline at EOF" markers and a banner for hidden whitespace-only changes. It also refactors image preview logic to handle Git LFS pointers and introduces a more descriptive styling system for DAG nodes, using diamonds for trunk bookmarks. Additionally, the commit message box now supports persistent drafts. Feedback was provided regarding a logic flaw in the EOF newline detection where context lines at the end of a file could cause markers to be incorrectly placed.

[chatgpt-codex-connector]

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