Phase 2: Split pane terminals, custom themes, and password strength meter

Split Pane Layout:
- Add SplitPaneManager with recursive tree-based pane splitting
- Support horizontal (Cmd+Shift+E) and vertical (Cmd+Shift+D) splits
- Draggable resize dividers with 100px minimum pane size
- Hover-visible close button to unsplit panes
- Session picker in new panes for connecting separate terminals
- Automatic xterm refit on all pane resize operations

Custom Terminal Themes:
- Add themes.js with 8 preset colour schemes: Charcoal (default),
  Solarized Dark, Monokai, Nord, Dracula, One Dark, Gruvbox Dark,
  Tokyo Night
- Add TerminalTheme preference to AppPreferences model
- Add colour scheme selector to Settings → Terminal tab
- Live-apply theme changes to all active terminals on save
- TerminalManager now uses themed colours instead of hardcoded values

Master Password Strength Meter:
- Add visual strength bar to vault creation dialog
- Entropy estimation based on length, character diversity, uniqueness
- Four levels: Weak (red), Fair (amber), Good (green), Strong (green)
- Animated transitions on strength changes
- Charcoal-themed CSS matching modal design

frontend/src/js/settings.js

@@ -73,6 +73,12 @@ class Settings {
         try {
             await window.go.api.Service.SavePreferences(this._prefs);
             this.app.state.setPreferences(this._prefs);
+
+            // Apply terminal colour theme change immediately.
+            if (this.app.terminals && this._prefs.terminalTheme) {
+                this.app.terminals.applyTheme(this._prefs.terminalTheme);
+            }
+
             this.app.notifications.success('Settings saved');
             this.close();
         } catch (err) {
@@ -205,6 +211,13 @@ class Settings {
      */
     _renderTerminalTab(body) {
         const p = this._prefs;
+
+        // Terminal colour theme selector
+        const themes = window.NexusTerminal.listTerminalThemes();
+        body.appendChild(this._settingField('select', 'terminalTheme', 'Colour scheme', p.terminalTheme || 'charcoal', {
+            options: themes,
+        }));
+
         body.appendChild(this._settingField('text', 'defaultFontFamily', 'Font family', p.defaultFontFamily, {
             placeholder: 'JetBrains Mono, Menlo, Monaco, Consolas, monospace',
         }));
@@ -455,7 +468,7 @@ class Settings {
         };
 
         const fields = [
-            'theme', 'defaultProtocol', 'confirmOnClose', 'confirmOnDisconnect',
+            'theme', 'terminalTheme', 'defaultProtocol', 'confirmOnClose', 'confirmOnDisconnect',
             'showStatusBar', 'sidebarWidth', 'defaultFontFamily', 'defaultFontSize',
             'defaultScrollback', 'defaultCursorStyle',
         ];