feat(v1.2.0): retry API, dernier commit, tri, coloration, export JSON

- client.py: _get_with_retry (max 2 retries, backoff lineaire), get_latest_commit
- collector.py: champ last_commit_date dans RepoData
- display.py: colonne "Dernier commit", _sort_repos (name/issues/release/activity),
  _colorize_milestone_due (rouge/jaune/vert selon echeance)
- cli.py: options --sort/-s et --format/-f (table/json)
- exporter.py: nouveau module, repos_to_dicts + export_json
- 88 tests (35 nouveaux), ruff clean

fixes #8, fixes #7, fixes #10, fixes #9, fixes #6

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/gitea_dashboard/collector.py

@@ -20,6 +20,7 @@ class RepoData:
     is_mirror: bool
     latest_release: dict | None  # {tag_name, published_at} ou None
     milestones: list[dict]  # [{title, open_issues, closed_issues, due_on}]
+    last_commit_date: str | None  # ISO 8601, ex: "2026-03-10T14:30:00Z"
 
 
 def _matches_any(name: str, patterns: list[str]) -> bool:
@@ -59,6 +60,9 @@ def collect_all(
         owner = repo["owner"]["login"]
         name = repo["name"]
 
+        commit = client.get_latest_commit(owner, name)
+        last_commit_date = commit["created"] if commit else None
+
         result.append(
             RepoData(
                 name=name,
@@ -70,6 +74,7 @@ def collect_all(
                 is_mirror=repo["mirror"],
                 latest_release=client.get_latest_release(owner, name),
                 milestones=client.get_milestones(owner, name),
+                last_commit_date=last_commit_date,
             )
         )