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>

models/repo/pull_request_default.go

@@ -0,0 +1,16 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repo
+
+import (
+	"context"
+
+	"code.gitea.io/gitea/models/unit"
+	"code.gitea.io/gitea/modules/util"
+)
+
+func (repo *Repository) GetPullRequestTargetBranch(ctx context.Context) string {
+	unitPRConfig := repo.MustGetUnit(ctx, unit.TypePullRequests).PullRequestsConfig()
+	return util.IfZero(unitPRConfig.DefaultTargetBranch, repo.DefaultBranch)
+}