[12.0][ADD] web_form_banner

[yostashiro]

The module adds configurable banners for backend form views. Define rules per model
(and optionally per view) to show context-aware alerts with a chosen severity (info/warning/danger).

Messages can be plain text with ${placeholders} or fully custom HTML; visibility,
severity, and values are computed server-side via a safe Python expression.

Banners are injected just before or after a target node (default: //sheet) and refresh
on form load/save/reload.

@qrtl QT5611

[AungKoKoLin1997]

Code and functional review: LGTM

[yostashiro]

@AungKoKoLin1997 Just added a commit since the message translation wasn't working. Also re-allocated some eval context vars from _get_banner_eval_context() to _banner_base_eval_ctx_static().

Please squash once you confirm this, and apply the same in the migration PR.

[yostashiro]

@AungKoKoLin1997 It was mistaken when I said the update didn't work. The updated code actually works fine.