🐛 fix(gui): resolve step header layout overlaps and improve scaling

- Fix 6 overlapping widget bugs in StepHeader (title/desc, btn/icon,
  run/rerun) with proper 5-column grid layout
- Merge run/rerun buttons into single _action_btn with state-driven text
- Fix DARK_CARD NameError (undefined import → STEP_CARD_BG)
- Replace fragile vertical connector lines with full-width dividers
- Add panel.grid_columnconfigure(0, weight=1) for horizontal scaling
- Unify button text across all states (▶ pending/disabled, ↻ retry)
- Tighten padding and reduce whitespace between steps

src/gui/main_window.py

@@ -42,7 +42,6 @@ from gui.styles import (
     DANGER_COLOR,
     INFO_COLOR,
     CONNECTOR_COLOR,
-    STEP_ICON_SIZE,
     get_theme,
 )
 from gui.widgets import (
@@ -189,9 +188,10 @@ class MainWindow(ctk.CTk):
         panel = ctk.CTkFrame(parent, corner_radius=CORNER_RADIUS)
         panel.grid(
             row=0, column=0, sticky="nsew",
-            padx=PADDING, pady=PADDING,
+            padx=PADDING, pady=(PADDING, 0),
         )
         panel.grid_rowconfigure(10, weight=1)
+        panel.grid_columnconfigure(0, weight=1)
 
         # ── Title ──
         title = ctk.CTkLabel(
@@ -199,7 +199,7 @@ class MainWindow(ctk.CTk):
             text="Workflow",
             font=FONT_HEADING,
         )
-        title.grid(row=0, column=0, padx=PADDING, pady=(PADDING, PADDING_SMALL))
+        title.grid(row=0, column=0, padx=PADDING_SMALL, pady=(PADDING_SMALL, PADDING_SMALL))
 
         # ── Steps ──
         step_defs = [
@@ -223,19 +223,18 @@ class MainWindow(ctk.CTk):
             self._steps.append(step)
             self._step_statuses[i] = "pending"
 
-            # Connector line between steps
+            # Divider line between steps
             if i > 0:
-                connector = ctk.CTkFrame(
+                divider = ctk.CTkFrame(
                     panel,
-                    width=3,
-                    height=12,
+                    height=1,
                     fg_color=CONNECTOR_COLOR,
-                    corner_radius=2,
                 )
-                connector.grid(
+                divider.grid(
                     row=i * 2, column=0,
-                    sticky="n",
-                    padx=(PADDING_SMALL + STEP_ICON_SIZE + PADDING_SMALL, PADDING_SMALL),
+                    sticky="ew",
+                    padx=PADDING,
+                    pady=(2, 0),
                 )
 
             # Position step
@@ -243,7 +242,7 @@ class MainWindow(ctk.CTk):
                 row=i * 2 + 1, column=0,
                 sticky="nsew",
                 padx=PADDING_SMALL,
-                pady=PADDING_SMALL,
+                pady=(0, 2),
             )
 
         # ── Progress indicator ──