feat: render screenshots using session theme palettes

Pass per-theme background, foreground, and 16-color palettes into the SVG
exporter so screenshots match the active session theme (including ANSI colors).

Adds theme palette mappings and updates screenshot tests to validate themed
backgrounds and palette-aware color conversion.

tests/test_svg_exporter.py

@@ -55,6 +55,30 @@ class TestColorToHex:
         """Color conversion covers named/hex/default cases."""
         assert _color_to_hex(color, is_foreground=is_foreground) == expected
 
+    def test_color_to_hex_uses_palette_defaults(self) -> None:
+        palette = {"red": "#123456"}
+        assert _color_to_hex(
+            "default",
+            is_foreground=True,
+            palette=palette,
+            default_fg="#111111",
+            default_bg="#222222",
+        ) == "#111111"
+        assert _color_to_hex(
+            "default",
+            is_foreground=False,
+            palette=palette,
+            default_fg="#111111",
+            default_bg="#222222",
+        ) == "#222222"
+        assert _color_to_hex(
+            "red",
+            is_foreground=True,
+            palette=palette,
+            default_fg="#111111",
+            default_bg="#222222",
+        ) == "#123456"
+
 
 class TestEscapeXml:
     """Tests for XML escaping."""