feat(branch): créer la branche depuis la carte (issue #3)

slugify + branch_name_for (issue-<n>-<slug>, reconnu par #1/#2);
client create_branch; POST /api/issues/{o}/{r}/{n}/branch (nom
libre validé, idempotent si existante); la carte passe en En
cours, journalisée source branch, étiquette synchronisée;
formulaire dans la section Code lié du panneau détail.

tests/conftest.py

@@ -38,6 +38,7 @@ def _repo(name: str, updated: datetime, archived: bool = False) -> dict:
         "owner": {"login": "Projets"},
         "archived": archived,
         "html_url": f"http://gitea.test/Projets/{name}",
+        "default_branch": "main",
     }
 
 
@@ -72,6 +73,7 @@ class FakeGitea:
             _repo("vieux", now - timedelta(days=800)),
             _repo("archive", now - timedelta(days=1), archived=True),
         ]
+        self.branches = ["main", "feature/1-tache-un", "issue-5-focus"]
         self.issues = {
             "Projets/demo": [
                 _issue(1, "Tâche un"),
@@ -224,11 +226,13 @@ class FakeGitea:
             self.label_ops.append(("remove", number, label["name"]))
 
     async def list_branches(self, owner, repo):
-        return [
-            {"name": "main"},
-            {"name": "feature/1-tache-un"},
-            {"name": "issue-5-focus"},
-        ]
+        return [{"name": n} for n in self.branches]
+
+    async def create_branch(self, owner, repo, new_branch, old_branch):
+        if new_branch in self.branches:
+            raise RuntimeError("branch exists")
+        self.branches.append(new_branch)
+        return {"name": new_branch}
 
     async def list_pulls(self, owner, repo, state="all"):
         return [