Allow configuring default PR base branch (fixes #36412) (#36425)

This adds a per-repository default PR base branch and wires it through
PR entry points. It updates compare links and recently pushed branch
prompts to respect the configured base branch, and prevents auto-merge
cleanup from deleting the configured base branch on same-repo PRs.

## Behavior changes
- New PR compare links on repo home/issue list and branch list honor the
configured default PR base branch.
- The "recently pushed new branches" prompt now compares against the
configured base branch.
- Auto-merge branch cleanup skips deleting the configured base branch
(same-repo PRs only).

---------

Signed-off-by: Louis <116039387+tototomate123@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

services/convert/repository.go

@@ -103,6 +103,7 @@ func innerToRepo(ctx context.Context, repo *repo_model.Repository, permissionInR
 	defaultDeleteBranchAfterMerge := false
 	defaultMergeStyle := repo_model.MergeStyleMerge
 	defaultAllowMaintainerEdit := false
+	defaultTargetBranch := ""
 	if unit, err := repo.GetUnit(ctx, unit_model.TypePullRequests); err == nil {
 		config := unit.PullRequestsConfig()
 		hasPullRequests = true
@@ -118,6 +119,7 @@ func innerToRepo(ctx context.Context, repo *repo_model.Repository, permissionInR
 		defaultDeleteBranchAfterMerge = config.DefaultDeleteBranchAfterMerge
 		defaultMergeStyle = config.GetDefaultMergeStyle()
 		defaultAllowMaintainerEdit = config.DefaultAllowMaintainerEdit
+		defaultTargetBranch = config.DefaultTargetBranch
 	}
 	hasProjects := false
 	projectsMode := repo_model.ProjectsModeAll
@@ -235,6 +237,7 @@ func innerToRepo(ctx context.Context, repo *repo_model.Repository, permissionInR
 		DefaultDeleteBranchAfterMerge: defaultDeleteBranchAfterMerge,
 		DefaultMergeStyle:             string(defaultMergeStyle),
 		DefaultAllowMaintainerEdit:    defaultAllowMaintainerEdit,
+		DefaultTargetBranch:           defaultTargetBranch,
 		AvatarURL:                     repo.AvatarLink(ctx),
 		Internal:                      !repo.IsPrivate && repo.Owner.Visibility == api.VisibleTypePrivate,
 		MirrorInterval:                mirrorInterval,