Add versionF sync workflow and bump to 0.19.0-beta.1

[ultraelectronica]

Summary

Prepares the 0.19.0-beta.1 release and automates keeping the versionF branch in sync with main.

versionF sync workflow (.github/workflows/sync-versionf.yml)

• Runs on every push to main, daily via schedule, and on manual dispatch.
• Uses the GitHub API to compare main...versionF; exits early when versionF is already up to date, otherwise merges main into versionF with Merge branch 'main' into versionF.
• After a merge, dispatches the CI workflow on versionF without requiring a local checkout, so the branch is validated automatically.
• Serialized with a sync-versionf concurrency group and minimal contents/actions write permissions.

Release bump to 0.19.0-beta.1 (+26)

• Updated pubspec.yaml, latchd version-stamp docs, Windows installer example, scripts/install-latchd.sh, and the README installer example.
• CHANGELOG.md now opens at 0.19.0-beta.1, with new Sharing & Smart Collections (Share to Latch, Smart Collections, shared file-filter model) and Operation Progress (shared operation gate, ToastHost) sections.
• whats_new.json gains a 0.19.0-beta.1 entry highlighting Recently Deleted, Share to Latch, Smart Collections, and Google Sans Flex.

Tooling

• Moves shadcn from dependencies to devDependencies and bumps it to 4.22.1; refreshes package-lock.json accordingly.

No app behavior changes beyond the release metadata and release notes.

Related issue

none

Checklist

• dart format --output=none --set-exit-if-changed lib test passes
• flutter analyze passes
• flutter test passes
• make latchd-test passes (if latchd/ or pocketbase/ changed)
• CHANGELOG.md updated under the current version heading
• Docs updated if behavior, setup, or scope changed (docs/ is the source of truth)
• No secrets, keys, keystores, or machine-local files committed
• No invariant violated (see CONTRIBUTING.md)