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

static/commands.js

@@ -124,7 +124,7 @@ async function cmdUsage(){
 }
 
 async function cmdTheme(args){
-  const themes=['dark','light','solarized','monokai','nord'];
+  const themes=['dark','slate','light','solarized','monokai','nord'];
   if(!args||!themes.includes(args.toLowerCase())){
     showToast('Usage: /theme '+themes.join('|'));
     return;