fix(github): append and edit review comments in an existing pending review

[matt2e]

Clicking GitHub on a review comment failed with a 422 (user_id can only have one pending review per pull request) whenever the user already had a draft review open on the PR, and the comment was lost. POST /pulls/{n}/comments creates a submitted one-comment review, so GitHub has to implicitly open a second pending review and refuses.

Posting

REST has no endpoint that appends to an existing pending review, so the inline path now checks for the viewer's draft up front and, when there is one, adds the comment with the GraphQL addPullRequestReviewThread mutation. Behaviour with no draft open is unchanged — the comment still lands immediately as a standalone submitted comment. The 422 is also retried through the append path, covering the race where a review is started between the lookup and the POST. A failed lookup is non-fatal and degrades to today's behaviour rather than blocking the post.

The comment ID comes from fullDatabaseId (a BigInt scalar, hence a JSON string), keeping the REST ID so the stored #discussion_r{id} anchor still resolves.

Editing

The REST review-comment endpoints do not list comments belonging to a draft review, and whether PATCH /pulls/comments/{id} reaches one anyway is undocumented. The edit path now works either way: PATCH is still tried first, so a published comment costs no extra round trip; a 404 on a review comment is treated as "possibly still a draft" and retried through GraphQL. Since GraphQL takes a node ID and only the REST database ID is persisted, the viewer's pending review is scanned (first 20 pending reviews, first 100 comments) for the matching fullDatabaseId. Not finding it — including when the draft was discarded, leaving the stored ID dangling — reports the original REST failure, which is the more useful error.

UI

A comment that joined a draft review is invisible to everyone else until the review is submitted, so GitHubCommentResult gains a non-persisted pending flag and the diff modal shows a toast instead of a silent checkmark.

Also in github.rs

• Add a shared graphql_request helper that surfaces errors[].message. GraphQL fails with HTTP 200 and an errors array, which neither existing call site checked, so failures deserialized into useless missing-field errors. update_pull_request is retrofitted onto it, replacing its hand-rolled quote escaping with proper JSON variables.
• Paginate fetch_pr_diff_lines. It read only the first default-size page of /pulls/{n}/files, so on a PR with more than 30 changed files, later files looked entirely outside the diff and an in-diff comment got a hard 422 instead of the inline post.
• Delete the unreachable sync_review_to_github and its helpers. It was never registered as a Tauri command, and its delete-then-recreate strategy is strictly worse than appending: it destroys the draft if the recreate fails and renumbers every comment.

Testing

Unit tests cover the 422 conflict matcher against the real response body, the mutation input for single-line vs multi-line spans, awkward comment bodies surviving as variables, fullDatabaseId parsing, GraphQL error extraction, Link header parsing, and the node-ID scan (match, another author's draft, empty/absent cases).

Verified: just fmt-check lint typecheck test test-frontend (653 Rust, 611 frontend).

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Recheck the PR head before adding to a pending review

When this pending-review path is used, the earlier current_head_sha check in post_single_comment_to_github is no longer tied to the actual write: addPullRequestReviewThread only receives path/line through this input, while the REST path still sends commit_id. If another client pushes to the PR after the head check but before this mutation, GitHub resolves the line against the newer head and the app records a successful comment that can be attached to the wrong line. Please revalidate the PR head immediately around this mutation and fail or delete the draft comment if it changed.

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