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>

custom/conf/app.example.ini

@@ -3064,6 +3064,8 @@ LEVEL = Info
 ;; Changes only apply to newly uploaded artifacts, existing ones keep the expiry stored when they were uploaded.
 ;; Artifacts could have their own retention periods by setting the `retention-days` option in `actions/upload-artifact` step.
 ;ARTIFACT_RETENTION_DAYS = 90
+;; Max artifact size in bytes that can be browsed or previewed in the web UI. Set to 0 to disable artifact previews, or -1 for no limit.
+;ARTIFACT_PREVIEW_MAX_SIZE = 10485760
 ;; Days to keep completed runs. Old runs and everything under them will be deleted after this period. 0 means keep forever.
 ;RUN_RETENTION_DAYS = 400
 ;; Timeout to stop the task which have running status, but haven't been updated for a long time