app launch pages: add Last updated line like base/runtime pages

[tengqm]

Context

Base and runtime pages carry a generated *Last updated: <ts> · <sha>* line
(see docs/gen_descriptions.py), but the app launch pages introduced in
PR #477 do not yet. This was deferred during review; app pages are generated
from the same docs/data/images.yaml + gen_descriptions.py pipeline, so the
line should come from the generator, not be hand-added.

Task

• Emit a Last updated line on app pages (web flavor), consistent in
  format and placement with base/runtime pages.
• Confirm whether the same line should appear in the Harbor plain flavor
  (PR 2, upload_descriptions.py --layer app).

Non-urgent: the app pages' content is regenerated from configs.yaml on every
gendoc run, so staleness risk is low. Tracked here so it is not forgotten.