v2.1.0

v2.1.0 - the OAuth fix users needed, plus lane scroll affordances

AUTH (the reason for this release): 'Connect via OAuth' failed on packaged Windows
installs with Bun's own EPERM reading the bundled omp inside the ACL-protected
application directory, instead of opening a sign-in page. NOT a 2.0.0 regression:
the offending resolver landed 2026-07-15 (c2d8cf9) and shipped in every tag from
v1.11.8 onward. Existence was never the question, runnability is: each candidate is
now probed by running it, with a fall-through and a log naming every path tried
when none work. Three files had private copies of that resolver and had already
drifted once; they now share one (11 tests).

FLEET: a lane transcript scrollbar with a real pointer target (the global thumb is
5px once its transparent border is accounted for), and the main composer's two
catch-up buttons per lane, with the scroll arithmetic shared by both surfaces so
the chat cannot get a tuning pass the lanes miss (11 tests).

Verified before tagging: CI green on master (harness tests, all three typecheck
passes, license headers, Python scanner) and CodeQL green. Gate 4921 pass with 7
standing Windows path-assumption failures.