fix(server): keep /api/diff selection state intact when diff parsing fails

[kiakiraki]

Summary

• move the currentSelection update in the /api/diff handler to after the diff is successfully resolved, so a request with an invalid ref no longer poisons the server state used as the fallback for subsequent requests
• return a JSON 500 error when diff parsing fails, matching the error handling pattern of the other API endpoints (previously Express's default HTML stack trace was returned)
• add a regression test covering both behaviors, replacing the previously skipped handles invalid commit gracefully placeholder

Root cause

In the /api/diff handler, currentSelection = requestedSelection; ran before parser.parseDiff(). When a request contained an invalid ref (e.g. /api/diff?target=deleted-branch — a branch deleted while switching revisions, or a hand-edited URL), the parse failed but the invalid selection had already overwritten currentSelection. Every subsequent parameter-less request (such as the initial fetch on page reload) then fell back to the poisoned selection and kept returning 500 until the server was restarted. The handler also had no try/catch, so the failure surfaced as Express's default HTML error page instead of JSON.

Reproduction (before this fix)

difit HEAD --no-open --port 4901 &
curl "http://localhost:4901/api/diff?target=nonexistent-branch"  # → 500 (HTML stack trace)
curl "http://localhost:4901/api/diff"                            # → 500 forever, until restart

Verification

• pnpm check / pnpm format / pnpm knip / pnpm test / pnpm build all pass
• confirmed the new regression test fails against the unfixed handler and passes with the fix
• manually verified with the built CLI: the first request now returns a JSON 500, and the following parameter-less request recovers with 200

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