refactor(ui): feature-gate models and utils, clean up cfg imports

The previous commit gated the TUI behind a feature flag but left the
models and utils modules unconditionally compiled. It turns out that
*every single consumer* of those modules is TUI-gated code — they
were compiling to dead code when building with --no-default-features.

Gate models and utils behind cfg(feature = "tui") where they belong.
While at it, consolidate the five separate #[cfg(feature = "tui")]
annotations on imports in workflow.rs into a single grouped use block,
because repeating the same attribute five times in a row is not my
idea of readability.

Also add a cargo check --no-default-features step to CI so this
kind of thing doesn't silently regress.

.github/workflows/build.yml

@@ -52,6 +52,12 @@ jobs:
           command: build
           args: --target ${{ matrix.target }}
 
+      - name: Check no-default-features
+        uses: actions-rs/cargo@v1
+        with:
+          command: check
+          args: --no-default-features --target ${{ matrix.target }}
+
       - name: Run tests
         uses: actions-rs/cargo@v1
         with:

crates/ui/src/handlers/workflow.rs

@@ -1,18 +1,16 @@
 // Workflow handlers
-#[cfg(feature = "tui")]
-use crate::app::App;
-#[cfg(feature = "tui")]
-use crate::models::{ExecutionResultMsg, WorkflowExecution, WorkflowStatus};
-#[cfg(feature = "tui")]
-use chrono::Local;
 use std::io;
 use std::path::{Path, PathBuf};
-#[cfg(feature = "tui")]
-use std::sync::mpsc;
-#[cfg(feature = "tui")]
-use std::thread;
 use wrkflw_evaluator::evaluate_workflow_file;
 use wrkflw_executor::{self, JobStatus, RuntimeType, StepStatus};
+#[cfg(feature = "tui")]
+use {
+    crate::app::App,
+    crate::models::{ExecutionResultMsg, WorkflowExecution, WorkflowStatus},
+    chrono::Local,
+    std::sync::mpsc,
+    std::thread,
+};
 
 // Validate a workflow or directory containing workflows
 pub fn validate_workflow(path: &Path, verbose: bool) -> io::Result<()> {

crates/ui/src/lib.rs

@@ -8,10 +8,8 @@
 // - utils: Contains utility functions
 // - views: Contains UI rendering code
 
-// Re-export public modules
+// Always-available modules (CLI validation/execution)
 pub mod handlers;
-pub mod models;
-pub mod utils;
 
 // TUI-specific modules (require ratatui/crossterm)
 #[cfg(feature = "tui")]
@@ -21,6 +19,10 @@ pub mod components;
 #[cfg(feature = "tui")]
 pub mod log_processor;
 #[cfg(feature = "tui")]
+pub mod models;
+#[cfg(feature = "tui")]
+pub mod utils;
+#[cfg(feature = "tui")]
 pub mod views;
 
 // Re-export main entry points