Refactor: extract edit UI HTML into bincio/edit/templates/edit.html

The 285-line _HTML string literal in edit/server.py is replaced by a
template file loaded at request time. The route handler is unchanged in
behaviour — it still substitutes __SITE_URL__, __SPORT_OPTIONS__, and
__STAT_CHECKBOXES__ before returning the response.

Five new tests cover: 200 response, form presence, site_url injection,
no unresolved placeholders, and template file existence on disk.

tests/test_edit_server.py

@@ -156,3 +156,34 @@ class TestDemEndpoint:
         monkeypatch.setattr(edit_server, "dem_url", "https://api.open-elevation.com")
         r = CLIENT.post("/api/activity/../../evil/recalculate-elevation/dem")
         assert r.status_code in (400, 404, 422)
+
+
+# ── /edit/{activity_id} HTML template ────────────────────────────────────────
+
+class TestEditPage:
+    AID = "2024-01-01T080000Z-my-ride"
+
+    def test_returns_200_html(self):
+        r = CLIENT.get(f"/edit/{self.AID}")
+        assert r.status_code == 200
+        assert r.headers["content-type"].startswith("text/html")
+
+    def test_contains_form(self):
+        r = CLIENT.get(f"/edit/{self.AID}")
+        assert '<form id="form"' in r.text
+
+    def test_site_url_placeholder_replaced(self, monkeypatch):
+        monkeypatch.setattr(edit_server, "site_url", "http://localhost:9999")
+        r = CLIENT.get(f"/edit/{self.AID}")
+        assert "http://localhost:9999" in r.text
+        assert "__SITE_URL__" not in r.text
+
+    def test_no_unresolved_placeholders(self):
+        r = CLIENT.get(f"/edit/{self.AID}")
+        for token in ("__SITE_URL__", "__SPORT_OPTIONS__", "__STAT_CHECKBOXES__"):
+            assert token not in r.text, f"Unresolved placeholder: {token}"
+
+    def test_template_file_exists_on_disk(self):
+        from pathlib import Path
+        template = Path(edit_server.__file__).parent / "templates" / "edit.html"
+        assert template.exists(), f"Template not found at {template}"