Allow configuring default Pull Request base branch (repo-level or user-level)

[tototomate123]

Problem

Gitea always defaults new Pull Requests to the repository default branch (usually main).
In repositories that use a long-lived integration branch (e.g. dev) and only merge to main for releases, this causes unnecessary friction.

Proposed solution

Add a configurable default PR base branch, either repository-level (e.g. "Default PR target branch") or user-level (preferred base branch per repo).

(This setting should only affect the initial PR creation UI and not change branch protection, merge rules, or existing behavior.)

This matches common workflows where main is protected and dev is the primary integration branch.

(Implemented in #36425)