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>

options/locale/locale_en-US.json

@@ -2124,6 +2124,8 @@
   "repo.settings.pulls.ignore_whitespace": "Ignore Whitespace for Conflicts",
   "repo.settings.pulls.enable_autodetect_manual_merge": "Enable autodetect manual merge (Note: In some special cases, misjudgments can occur)",
   "repo.settings.pulls.allow_rebase_update": "Enable updating pull request branch by rebase",
+  "repo.settings.pulls.default_target_branch": "Default target branch for new pull requests",
+  "repo.settings.pulls.default_target_branch_default": "Default branch (%s)",
   "repo.settings.pulls.default_delete_branch_after_merge": "Delete pull request branch after merge by default",
   "repo.settings.pulls.default_allow_edits_from_maintainers": "Allow edits from maintainers by default",
   "repo.settings.releases_desc": "Enable Repository Releases",
@@ -2436,9 +2438,10 @@
   "repo.settings.block_outdated_branch_desc": "Merging will not be possible when head branch is behind base branch.",
   "repo.settings.block_admin_merge_override": "Administrators must follow branch protection rules",
   "repo.settings.block_admin_merge_override_desc": "Administrators must follow branch protection rules and cannot circumvent it.",
-  "repo.settings.default_branch_desc": "Select a default repository branch for pull requests and code commits:",
+  "repo.settings.default_branch_desc": "Select a default branch for code commits.",
+  "repo.settings.default_target_branch_desc": "Pull requests can use different default target branch if it is set in the Pull Requests section of Repository Advance Settings.",
   "repo.settings.merge_style_desc": "Merge Styles",
-  "repo.settings.default_merge_style_desc": "Default Merge Style",
+  "repo.settings.default_merge_style_desc": "Default merge style",
   "repo.settings.choose_branch": "Choose a branch…",
   "repo.settings.no_protected_branch": "There are no protected branches.",
   "repo.settings.edit_protected_branch": "Edit",
@@ -2650,7 +2653,7 @@
   "repo.branch.restore_success": "Branch \"%s\" has been restored.",
   "repo.branch.restore_failed": "Failed to restore branch \"%s\".",
   "repo.branch.protected_deletion_failed": "Branch \"%s\" is protected. It cannot be deleted.",
-  "repo.branch.default_deletion_failed": "Branch \"%s\" is the default branch. It cannot be deleted.",
+  "repo.branch.default_deletion_failed": "Branch \"%s\" is the default or pull request target branch. It cannot be deleted.",
   "repo.branch.default_branch_not_exist": "Default branch \"%s\" does not exist.",
   "repo.branch.restore": "Restore Branch \"%s\"",
   "repo.branch.download": "Download Branch \"%s\"",