Update repository: improved Browser Build, update fastlane

[marionbarker]

Purpose

The repository must be updated at least once every 60 days to prevent GitHub from disabling the automatic build action.

When doing this, look for reasonable changes to make.

1. Update fastlane to same version as LoopWorkspace
2. Adopt the LoopFollow method of explicitly skipping an automatic build on weeks when no build is completed (details below), and update Xcode version to 26.5, same as is used in LoopWorkspace

Details

The scheduled weekly run checks for new commits and only builds when something changed (or when the monthly forced build is due). When there is nothing to build, the build jobs are skipped and the run still completes with a green check — indistinguishable at a glance from a successful build. Users who rely on the green status can believe they have a fresh build even when their last real build attempt failed.

Copied the change from loopandlearn/LoopFollow#705. See that PR for more information.

Status semantics after this change:

green — a new build was made and uploaded to TestFlight
cancelled (grey) — the repository was already up to date, nothing was built
red — a build was attempted and failed