fix: settings unsaved-changes guard, add Slate theme, improve Light theme

Unsaved-changes guard:
- _closeSettingsPanel() intercepts all three close paths (X button, overlay
  click, Escape key) and checks _settingsDirty before closing
- If dirty: shows inline 'Unsaved changes' bar with Save & Close / Discard
- Discard reverts the live theme preview to what it was when panel opened
- _markSettingsDirty() wired to all inputs via addEventListener in loadSettingsPanel()
- saveSettings() now resets dirty flag and hides the bar on successful save

Theme improvements:
- Add 'Slate' theme: warm charcoal (#2b2d30 bg), a softer/lighter dark option
  that sits between Dark and the full light themes
- Rework 'Light' theme: replace pure white (#f5f5f7) with warm off-white
  (#f0ede8) -- warmer, lower contrast, less harsh on most displays
- Update /theme command to include 'slate' in valid list
- Add test_settings_set_theme_slate() to test_sprint26.py

tests/test_sprint26.py

@@ -74,6 +74,16 @@ def test_settings_set_theme_nord():
         post("/api/settings", {"theme": "dark"})
 
 
+def test_settings_set_theme_slate():
+    """Setting theme to 'slate' should persist."""
+    try:
+        post("/api/settings", {"theme": "slate"})
+        d, _ = get("/api/settings")
+        assert d.get("theme") == "slate"
+    finally:
+        post("/api/settings", {"theme": "dark"})
+
+
 def test_settings_custom_theme_accepted():
     """Custom theme names should be accepted (no enum gate)."""
     try: