feat(actions): Add artifact preview in Actions run view (#36754)

Closes https://github.com/go-gitea/gitea/issues/33579.

Adds browser previews for Actions artifacts. Selecting an artifact opens
its file browser; selecting a file renders it in the same tab. The ZIP
download remains available separately.

Previews require sign-in and read access to the run. Text, image and PDF
files are supported; rendered HTML and JavaScript run in a sandboxed
frame and are labeled as automatically generated. The frame loads files
from a signed link that expires after an hour, because its requests
carry no session cookie. `[actions] ARTIFACT_PREVIEW_MAX_SIZE` limits
total previewable artifact size (`0` disables previews; `-1` removes the
limit); individual files also follow `[ui] MAX_DISPLAY_FILE_SIZE`.

<img width="1803" height="913" alt="image"
src="https://github.com/user-attachments/assets/a38fd704-2244-44fa-9181-c695ecbe0276"
/>

Docs: https://gitea.com/gitea/docs/pulls/533

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>

modules/setting/actions.go

@@ -43,6 +43,8 @@ var (
 		// transaction at once per Gitea instance, to avoid a thundering herd when many
 		// runners poll together. It is a per-process limit, not a cluster-wide one.
 		MaxConcurrentTaskPicks int `ini:"MAX_CONCURRENT_TASK_PICKS"`
+
+		ArtifactPreviewMaxSize int64 `ini:"ARTIFACT_PREVIEW_MAX_SIZE"`
 	}{
 		Enabled:                true,
 		DefaultActionsURL:      defaultActionsURLGitHub,
@@ -51,6 +53,7 @@ var (
 		ScopedWorkflowDirs:     []string{".gitea/scoped_workflows"},
 		MaxRerunAttempts:       defaultMaxRerunAttempts,
 		MaxConcurrentTaskPicks: defaultMaxConcurrentTaskPicks,
+		ArtifactPreviewMaxSize: 10 * 1024 * 1024,
 		LogRetentionDays:       defaultLogRetentionDays,
 		ArtifactRetentionDays:  defaultArtifactRetentionDays,
 		RunRetentionDays:       defaultRunRetentionDays,