fix: render Jinja2 watch URL on diff/preview pages instead of the raw template (#3776)

[ebarkhordar]

Fixes #3776

Root cause

The diff and preview pages pass the raw watch['url'] into base.html as current_diff_url:

• changedetectionio/processors/text_json_diff/difference.py (diff page)
• changedetectionio/processors/image_ssim_diff/difference.py (image diff page)
• changedetectionio/blueprint/ui/preview.py (preview page)
• changedetectionio/processors/image_ssim_diff/preview.py (image preview page)

base.html then renders it as <a class="current-diff-url" href="{{ current_diff_url }}">. When a watch URL contains a Jinja2 template such as {% now 'utc', '%Y' %}, the link points at the unrendered template string, so it shows raw and is not clickable (the reported behaviour).

Invariant

The URL linked on the diff and preview pages must be the Jinja2-rendered, safety-validated Watch.link value, never the raw template. Watch.link (model/Watch.py) already does jinja_render() plus is_safe_valid_url() validation and is what the watch list uses, so the diff and preview headers should use the same value.

Fix

Replace current_diff_url=watch['url'] with current_diff_url=watch.link at the four render call sites above. For a plain URL Watch.link returns it unchanged, so nothing changes for non-templated watches; for an invalid URL it returns DISABLED, which base.html already guards with is_safe_valid_url(current_diff_url).

The reported issue is the diff page; the same raw-template value was passed at the preview and image-processor render calls, so I fixed all four rather than leave the preview page showing the same unrendered URL.

Verification

Ran in a clean Docker container (python:3.11-slim, requirements.txt), against this branch and against master:

• New test tests/test_diff_url_jinja2_render.py (adds a paused watch with a Jinja2 URL, injects two history snapshots, loads the diff page): fails on master (the current-diff-url href is the raw ...{% now ... %}...), passes on this branch (href is the rendered URL). Confirmed both runs load the branch/master copy via module.__file__.
• Existing tests/test_download_patch.py (renders the diff page for normal URLs): 3 passed on this branch, so watch.link does not change plain-URL rendering.

What I did not verify: tests/test_jinja2.py::test_jinja2_in_url_query (preview page, does a live fetch) could not run in my container because the fetcher could not resolve the live-server host; it fails the same way on unmodified master, so it is an environment limit on my side, not this change. The preview and image-processor call sites get the same one-line substitution and the same base.html contract as the diff page, but only the diff page has a new regression test here.

AI assistance: this fix was prepared with AI help; the reproduction, the differential test, and the Docker runs above were executed and checked before opening.