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/index.html

@@ -16,6 +16,7 @@
     <link rel="stylesheet" href="/src/css/sidebar.css">
     <link rel="stylesheet" href="/src/css/tabs.css">
     <link rel="stylesheet" href="/src/css/terminal.css">
+    <link rel="stylesheet" href="/src/css/splitpane.css">
     <link rel="stylesheet" href="/src/css/forms.css">
     <link rel="stylesheet" href="/src/css/modals.css">
     <link rel="stylesheet" href="/src/css/github.css">
@@ -168,6 +169,7 @@
     <script src="/src/js/notifications.js"></script>
     <script src="/src/js/sidebar.js"></script>
     <script src="/src/js/tabs.js"></script>
+    <script src="/src/js/themes.js"></script>
     <script src="/src/js/terminal.js"></script>
     <script src="/src/js/profiles.js"></script>
     <script src="/src/js/raw.js"></script>
@@ -175,6 +177,7 @@
     <script src="/src/js/settings.js"></script>
     <script src="/src/js/commands.js"></script>
     <script src="/src/js/secrets.js"></script>
+    <script src="/src/js/splitpane.js"></script>
     <script src="/src/js/app.js"></script>
 </body>
 </html>