fix: simplify Docker build workflow, remove PREBUILT artifact workaround

- Remove UI build artifact upload/download steps (18 lines removed)
- Remove .dockerignore manipulation workaround
- Always build UI from source inside Docker (more reliable)
- Remove PREBUILT arg from ui/Dockerfile and docker-bake.hcl

This fixes the '/app/build not found' error in CI by eliminating the fragile
artifact-passing mechanism. UI now builds fresh in Docker using build cache,
same as local development.

docker-bake.hcl

@@ -1,13 +1,11 @@
 # docker-bake.hcl — defines all five production images.
 #
-# Uses only plain variables for broad buildx compatibility (no locals/functions).
-# CI pre-computes VERSION and MAJOR_MINOR from the git tag and passes them as
-# env vars.  Locally, everything gets a :dev tag.
+# CI passes version info as environment variables; locally everything gets :dev tags.
 #
 # Local build (no push):
 #   docker buildx bake
 #
-# CI passes: DOCKER_USER, VERSION, MAJOR_MINOR, COMMIT, BUILD_TIME
+# CI environment variables: VERSION, MAJOR_MINOR, COMMIT, BUILD_TIME
 
 variable "DOCKER_USER"  { default = "kalekber" }
 variable "VERSION"      { default = "dev" }    # e.g. "4.1.6"  (no leading v)
@@ -85,7 +83,6 @@ target "ui" {
     BUILD_VERSION = VERSION
     BUILD_COMMIT  = COMMIT
     BUILD_TIME    = BUILD_TIME
-    PREBUILT      = "1"
   }
 }