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/index.html

@@ -312,7 +312,7 @@
   <div class="settings-panel">
     <div class="settings-header">
       <h3 style="margin:0;font-size:16px">Settings</h3>
-      <button class="panel-icon-btn" onclick="toggleSettings()" title="Close">&#10005;</button>
+      <button class="panel-icon-btn" onclick="_closeSettingsPanel()" title="Close">&#10005;</button>
     </div>
     <div class="settings-body">
       <div class="settings-field">
@@ -334,7 +334,8 @@
         <label for="settingsTheme">Theme</label>
         <select id="settingsTheme" style="width:100%;padding:8px;background:var(--code-bg);color:var(--text);border:1px solid var(--border2);border-radius:6px" onchange="document.documentElement.dataset.theme=this.value;localStorage.setItem('hermes-theme',this.value)">
           <option value="dark">Dark (default)</option>
-          <option value="light">Light</option>
+          <option value="slate">Slate (charcoal)</option>
+          <option value="light">Light (warm off-white)</option>
           <option value="solarized">Solarized Dark</option>
           <option value="monokai">Monokai</option>
           <option value="nord">Nord</option>