Phase 0: remove auto-updater, pin Go 1.27.1, fix vet warnings, tidy ignores

- Delete BinaryUpdate, internal/up2date, GitHub/Update structs and the
  xteveAutoUpdate / update.url settings (UI rows, en.json, defaults).
  Settings-schema migrations kept and moved to src/migrate.go.
- Drop kardianos/osext dependency.
- xteve.go version 0200 -> 0201 to match changelog; Drone now fails on drift.
- go.mod go 1.27.1; Dockerfile and Drone golang images pinned to 1.27.1.
- Fix four go vet unreachable-code warnings.
- .gitignore: .gocache/, agent.md, skill.md. .dockerignore: build context
  no longer includes caches, ts/, tasks/ or markdown except the changelog.
- Drone: publish :latest only from master; other branches publish a
  branch-named tag so a feature push cannot replace the deployed image.
- Add tasks/improvement-plan.md and tasks/todo.md.
- Regenerate src/webUI.go.

html/js/settings_ts.js

@@ -241,17 +241,6 @@ var SettingsCategory = /** @class */ (function () {
                 setting.appendChild(tdLeft);
                 setting.appendChild(tdRight);
                 break;
-            case "xteveAutoUpdate":
-                var tdLeft = document.createElement("TD");
-                tdLeft.innerHTML = "{{.settings.xteveAutoUpdate.title}}" + ":";
-                var tdRight = document.createElement("TD");
-                var input = content.createCheckbox(settingsKey);
-                input.checked = data;
-                input.setAttribute("onchange", "javascript: this.className = 'changed'");
-                tdRight.appendChild(input);
-                setting.appendChild(tdLeft);
-                setting.appendChild(tdRight);
-                break;
             case "api":
                 var tdLeft = document.createElement("TD");
                 tdLeft.innerHTML = "{{.settings.api.title}}" + ":";
@@ -386,9 +375,6 @@ var SettingsCategory = /** @class */ (function () {
                     text = "{{.settings.authenticationAPI.description}}";
                 }
                 break;
-            case "xteveAutoUpdate":
-                text = "{{.settings.xteveAutoUpdate.description}}";
-                break;
             case "backup.keep":
                 text = "{{.settings.backupKeep.description}}";
                 break;