Terminal maximize (full editor coverage)

[JDonaghy]

Toggle the terminal panel to cover the entire editor area (like VSCode's "Maximize Panel Size" / Ctrl+` behavior).

Behavior:

• When maximized, the terminal fills the space normally occupied by editor panes
• Sidebar remains visible
• A second toggle restores the previous layout
• Keyboard shortcut + toolbar button

Backends: GTK, TUI, and Win-GUI

Key files: Terminal panel rendering in each backend, layout calculation in render.rs

[JDonaghy]

Implemented in 45511d7..5334fac (11 commits, merged to develop via fast-forward). Ctrl-Shift-T / :TerminalMaximize / toolbar button across TUI, GTK, and Win-GUI; render-time effective-rows design so window resize is handled automatically.