- Add serializable state structs (WorkspaceState, TabState, PaneState, etc.)
- Implement Workspace::save() to persist open tabs, cursor positions,
  viewport scroll, and pane bounds to .fackr/workspace.json
- Implement Workspace::load() to restore workspace state on startup,
  handling missing files gracefully
- Add Cursors::from_cursor() constructor for state restoration
- Call workspace.save() on editor exit
- Replace empty default tab when opening explicit file to avoid
  spurious [new] tab appearing as tab 1

Add active window focus system where keyboard input is routed to the
last-activated component instead of using fixed priority.

Changes:
- Add Focus enum (Editor, Terminal, FussMode, ServerManager, Prompt)
- Add HitRegion enum for mouse hit testing
- Add focus field to Editor struct
- Implement hit_test() to determine clicked region
- Implement return_focus() helper for closing components
- Add click-to-focus in mouse handler
- Update toggle handlers to set focus when opening/closing
- Refactor keyboard routing from priority-based to focus-based

Behavior:
- Clicking any component gives it keyboard focus
- Keyboard shortcuts that open components also focus them
- Clicking in split panes makes that pane active
- ESC closes focused component and returns focus sensibly

Fill the status bar row in fuss mode column with background color
to prevent terminal content from showing through.

Add methods to determine which pane contains a screen coordinate:
- Tab::pane_at_screen_position() - finds pane by normalized coordinates
- Workspace::pane_at_position() - wrapper for active tab

When both fuss mode and terminal are visible, fuss mode now only renders
above the terminal instead of using full screen height. This prevents
visual artifacts where fuss mode content would render into the terminal
area.

- Add max_rows parameter to render_fuss
- Calculate available height based on terminal start row

- F3/Ctrl+B now toggles fuss mode even when terminal is focused
- Terminal shrinks to fit alongside fuss mode instead of overlapping
- Pass left_offset to render_terminal for proper positioning

Fuss mode (file tree) should be accessible even when terminal is active.
Add pass-through for F3 and Ctrl+B keybindings so they toggle fuss mode
instead of being sent to the terminal.

Add keybindings for terminal tab management (when terminal focused):
- Alt+T: Create new terminal tab
- Alt+Q: Close current tab (hides terminal if last)
- Alt+.: Next tab
- Alt+,: Previous tab
- Alt+1-9: Switch to specific tab

Update terminal title bar to show tabs when multiple sessions exist:
- Single session: show CWD or "Terminal" centered with separators
- Multiple sessions: tab bar with numbers and directory names
- Active tab highlighted with bold white on darker background
- Add extract_dirname() helper for path display

Add CWD tracking to TerminalScreen:
- Add cwd field to store current working directory
- Implement osc_dispatch to parse OSC 7 sequences
- Extract path from file://hostname/path format
- Fish shell emits OSC 7 automatically on directory change

Refactor TerminalPanel to support multiple terminal sessions:
- Add TerminalSession struct holding PTY + screen buffer per session
- Change from single PTY to Vec<TerminalSession> with active index
- Add session management methods: new_session(), close_active_session(),
  switch_session(), next_session(), prev_session()
- Poll all sessions to keep background shells responsive
- Auto-remove dead sessions and hide terminal when last closes

Instead of sending color/attribute escape sequences for every cell,
track current state and only send changes. Batch consecutive characters
with same attributes into single print operations. This dramatically
reduces the number of escape sequences sent per render.

Match the pattern of other render functions by hiding cursor at start
of render_terminal() before showing it at the positioned location.

Detect when the shell process exits (via EOF or error on PTY read)
and automatically hide the terminal panel. Typing 'exit' now cleanly
closes the integrated terminal instead of leaving it in a broken state.

The PTY and screen buffer now use height-1 to account for the
title bar row, so multi-line prompts (like fish) display fully.

- Fix flashing: poll() now returns bool, only render when data received
- Add DEC private modes: ?1/?7/?25/?1049/?2004 (cursor keys, autowrap,
  cursor visibility, alternate screen, bracketed paste)
- Implement alternate screen buffer for vim/less compatibility
- Add missing CSI commands: E/F/G/L/M/P/S/T/X/@/d/n/r/s/u
- Complete J=1 and K=1 (clear from start to cursor)
- Add scroll region support (DECSTBM)
- Handle escape sequences: ESC 7/8/M/D/E/c (cursor save/restore,
  index, reverse index, next line, reset)
- Add device status report responses (CSI 5n, CSI 6n)

This should fix fish shell's two-line prompt and vim rendering.

Ctrl+` can conflict with tmux prefix mappings, so Ctrl+j
provides a reliable alternative for toggling the terminal.

Implement basic integrated terminal with:
- Ctrl+` to toggle terminal visibility
- ESC to hide terminal when focused
- PTY spawning using $SHELL environment variable
- VTE-based escape sequence parsing (colors, cursor, SGR)
- Terminal renders at bottom of screen (30% default height)
- Shell process persists when terminal is hidden
- Full input routing (control chars, arrows, function keys)

New modules:
- terminal/pty.rs: PTY spawning and I/O with reader thread
- terminal/screen.rs: Terminal buffer with VTE Perform impl
- terminal/panel.rs: Terminal panel visibility and lifecycle