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.

.drone.yml

@@ -4,16 +4,18 @@ name: default
 
 steps:
   - name: go-test
-    image: cache.coadcorp.com/library/golang
+    image: cache.coadcorp.com/library/golang:1.27.1
     commands:
       - go mod download
       - go test ./...
 
   - name: go-build
-    image: cache.coadcorp.com/library/golang
+    image: cache.coadcorp.com/library/golang:1.27.1
     commands:
       - XTEVE_VERSION="$(grep -m1 '^#### ' changelog-beta.md | cut -d' ' -f2 | sed 's/-beta$//')"
       - test -n "$XTEVE_VERSION" || (echo "Could not parse version from changelog-beta.md" && exit 1)
+      - SRC_VERSION="$(grep -m1 '^var Version = ' xteve.go | cut -d'"' -f2)"
+      - test "$SRC_VERSION" = "$XTEVE_VERSION" || (echo "Version drift: xteve.go has $SRC_VERSION, changelog-beta.md has $XTEVE_VERSION" && exit 1)
       - echo "Building xTeVe version $XTEVE_VERSION from changelog-beta.md"
       - go build -v -ldflags "-X main.Version=$XTEVE_VERSION" ./...
 
@@ -61,3 +63,33 @@ steps:
     when:
       event:
         - push
+      branch:
+        - master
+
+  - name: docker-publish-branch
+    image: plugins/docker
+    environment:
+      XTEVE_UID: "1000"
+      XTEVE_GID: "1000"
+    settings:
+      registry: registry.coadcorp.com
+      repo: registry.coadcorp.com/nathan/xteve
+      dockerfile: Dockerfile
+      username: nathan
+      password:
+        from_secret: registry_password
+      tags:
+        - ${DRONE_BRANCH}
+        - ${DRONE_COMMIT_SHA}
+      build_args:
+        - TARGETOS=linux
+        - TARGETARCH=amd64
+      build_args_from_env:
+        - XTEVE_UID
+        - XTEVE_GID
+    when:
+      event:
+        - push
+      branch:
+        exclude:
+          - master