App template on qcobjects 2.7.3.

- require qcobjects >=2.7.3 <3.0.0; lockfile refreshed from the pinned
  2.7.2 so the template receives the template-processor and bundled-SDK
  fixes
- qcobjects-sdk and qcobjects-cli remain at 2.7.2, unchanged and
  compatible

Verified against the published packages: 6 specs 0 failures, lint 0
errors, and a headless Chromium run showing 145 components built (was 25)
with 60 github-card components rendered from live GitHub results, no
literal $mapper in the DOM and no QCObjects-SDK import warning.