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

internal/models/models.go

@@ -230,6 +230,7 @@ type KnownHost struct {
 // AppPreferences stores user-level application settings.
 type AppPreferences struct {
 	Theme               string `json:"theme"`
+	TerminalTheme       string `json:"terminalTheme"` // colour scheme name (e.g. "charcoal", "solarized-dark")
 	DefaultProtocol     string `json:"defaultProtocol"`
 	ConfirmOnClose      bool   `json:"confirmOnClose"`
 	ConfirmOnDisconnect bool   `json:"confirmOnDisconnect"`