The app links to its source, from an admin setting (#6228)

#6228 "The app offers its source: a Source link from an admin setting
(AGPL §13)". The AGPL requires a modified copy run for others over a
network to offer them its source; Forge's #2282 is the shape followed.

- Settings → General gains "Source code" (`source_url`), defaulting to
  the public repo so a stock install makes the offer. Empty hides it.
- It is an http(s)-only URL setting, refused otherwise on save, and
  `safe_link` refuses anything else again when it is published, because
  the web renders it as an href.
- /api/config carries it; the web shows "Source" beside the build number
  at the foot of the sidebar. The offline desktop's config has no such
  field, so the link stays hidden there.
- ci.yml's client-bake comments no longer call the forge private; the
  repo has been public since #2091.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_retention.py

@@ -66,6 +66,8 @@ async def test_public_config_publishes_the_window():
 
     cfg = await get_public_config(_NoRows())
     assert cfg["trash_retention_days"] == 30
+    # The AGPL source offer is on by default, so a stock install makes it.
+    assert cfg["source_url"] == "https://git.fabledsword.com/bvandeusen/inkwell"
 
 
 @pytest.mark.parametrize(