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_test.go

@@ -0,0 +1,32 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repo
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/unit"
+	"code.gitea.io/gitea/models/unittest"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDefaultTargetBranchSelection(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	ctx := t.Context()
+	repo := unittest.AssertExistsAndLoadBean(t, &Repository{ID: 1})
+
+	assert.Equal(t, repo.DefaultBranch, repo.GetPullRequestTargetBranch(ctx))
+
+	repo.Units = nil
+	prUnit, err := repo.GetUnit(ctx, unit.TypePullRequests)
+	assert.NoError(t, err)
+	prConfig := prUnit.PullRequestsConfig()
+	prConfig.DefaultTargetBranch = "branch2"
+	prUnit.Config = prConfig
+	assert.NoError(t, UpdateRepoUnit(ctx, prUnit))
+	repo.Units = nil
+	assert.Equal(t, "branch2", repo.GetPullRequestTargetBranch(ctx))
+}