feat: update scroll.py to use theme message gradient

Add msg_gradient() helper function to render.py that applies message
(ntfy) gradient using the active theme's message_gradient property.
This replaces hardcoded magenta gradient in scroll.py with a theme-aware
approach that uses complementary colors from the active theme.

- Add msg_gradient(rows, offset) helper in render.py with fallback to
  default magenta gradient when no theme is active
- Update scroll.py imports to use msg_gradient instead of
  lr_gradient_opposite
- Replace lr_gradient_opposite() call with msg_gradient() in message
  overlay rendering
- Add 6 comprehensive tests for msg_gradient covering theme usage,
  fallback behavior, and edge cases

All tests pass (121 passed), no regressions detected.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

engine/render.py

@@ -254,6 +254,29 @@ def lr_gradient_opposite(rows, offset=0.0):
     return lr_gradient(rows, offset, _default_magenta_gradient())
 
 
+def msg_gradient(rows, offset):
+    """Apply message (ntfy) gradient using theme complementary colors.
+
+    Returns colored rows using ACTIVE_THEME.message_gradient if available,
+    falling back to default magenta if no theme is set.
+
+    Args:
+        rows: List of text strings to colorize
+        offset: Gradient offset (0.0-1.0) for animation
+
+    Returns:
+        List of rows with ANSI color codes applied
+    """
+    from engine import config
+
+    cols = (
+        _color_codes_to_ansi(config.ACTIVE_THEME.message_gradient)
+        if config.ACTIVE_THEME
+        else _default_magenta_gradient()
+    )
+    return lr_gradient(rows, offset, cols)
+
+
 # ─── HEADLINE BLOCK ASSEMBLY ─────────────────────────────
 def make_block(title, src, ts, w):
     """Render a headline into a content block with color."""