Embedded Terminal tool window (JetBrains-style), rooted at the workspace folder

[dG9hc3Q]

Summary

Add an embedded Terminal tool window, like JetBrains' terminal: a
bottom-docked panel running a real interactive shell rooted at the current
workspace folder, so the writer can run git, scripts, file ops etc.
without leaving Rhymr.

Motivation

Rhymr is aiming to be a "JetBrains-capable IDE" for lyrics; an integrated
terminal is table stakes for that workflow. It also complements the VCS
work (gutter, branch status, Git Log #3) — quick git / shell access in the
project context.

Behaviour (target)

• Bottom-docked tool window, same mechanism as Rhyme Search / Git Log —
  registers as a ToolWindow in the dock manager (Rearrangeable tool-window layout: dock/move/resize/float like a JetBrains IDE #4) so it can be
  toggled, resized, moved and (eventually) floated.
• Toggled from the bottom stripe + an app.terminal gio action/accel
  (JetBrains uses Alt+F12) + a menu entry.
• CWD = workspace root. Uses the shared root path from
  WorkspaceController; when there's no open project, falls back to the
  user's home dir. Opening / switching projects starts (or offers to
  restart) the shell in the new root — mirror how the status-bar branch
  segment and file tree react to a root change.
• Real interactive PTY: full-screen curses apps (vim, htop, git log
  pager), colour, resize/SIGWINCH, Ctrl-C etc. all work.
• Multiple terminal tabs within the panel, each independently
  rooted at the workspace; a "+" to open another, close on tab close or
  shell exit.
• Shell selection: Settings override, else $SHELL on macOS / %ComSpec%
  (or PowerShell) on Windows.
• Copy/paste, select, scrollback, clear; font = the editor monospace
  (--app-font-* from Settings); 16-colour + default fg/bg from the
  src/css.rs PALETTE (dark/light), matched to the editor scheme.
• PTY read/write loop runs off the GTK main thread; output marshalled
  to the widget on the main loop (CLAUDE.md non-negotiable).

Key design question — terminal widget / cross-platform parity

CLAUDE.md requires macOS + Windows parity and a shared platform trait,
not copy-paste. The terminal core is the crux:

• Option A — VTE4 (vte4 crate / libvte-2.91-gtk4): drop-in GTK4
  terminal widget, least code. But VTE is GNOME/Linux-first — awkward to
  ship on macOS (Homebrew libvte) and not available on Windows. Fails
  the parity requirement on its own.
• Option B — portable PTY + emulator core + custom render (recommended
  for parity): portable-pty (wezterm) for the cross-platform PTY
  (ConPTY on Windows, openpty on Unix), an emulation core
  (alacritty_terminal, or the vte parser crate feeding our own grid),
  rendered into a gtk::DrawingArea with the Settings monospace font and
  palette colours. More work, but one code path on both OSes with only the
  PTY spawn behind the platform trait.
• Option C — hybrid: VTE4 where present, Option B elsewhere. Two
  renderers to maintain — least attractive.

Recommend Option B. Confirm approach before starting (this touches the
platform trait — per CLAUDE.md "when unsure, ask").

Proposed structure

• app/terminal.rs — the panel widget: tab bar + terminal view(s),
  ToolWindow registration, toggle plumbing.
• terminal/ module — OS-agnostic: PTY session lifecycle, the emulator
  grid/parser, input encoding, resize.
• platform/* — only the PTY spawn (shell path, env, ConPTY vs openpty)
  behind the existing platform trait; no duplicated session logic.
• Constants (default shell fallbacks, scrollback limit, read-buffer size)
  in the one constants/config module, not scattered.
• SCSS stem for the panel/tab-bar chrome, registered in CSS_FILES in
  src/css.rs and build.rs; boxy, --radius-* = 0. Icons via
  crate::app::icons::img.
• Persist open/closed (and tab count / cwd if cheap) via the layout.*
  settings area from Rearrangeable tool-window layout: dock/move/resize/float like a JetBrains IDE #4.

Phasing

• P1 — single terminal, PTY rooted at workspace root, basic
  colour/resize/scrollback, copy-paste, off-thread IO. Depends on the
  widget decision above.
• P2 — multiple tabs; shell selection setting; restart-on-root-change;
  clear action.
• P3 — theming polish (palette-matched colours, cursor styles), search
  in scrollback, "open terminal here" from a file-tree folder context
  menu.

Out of scope

• A full multiplexer / split panes inside one terminal tab.
• Remote / SSH sessions.
• Running Rhymr build tasks through a task-runner UI (separate concern).

Acceptance criteria

• Bottom-docked "Terminal" tool window via the dock manager (Rearrangeable tool-window layout: dock/move/resize/float like a JetBrains IDE #4),
  toggled from the stripe + app.terminal action/accel + menu entry.
• Interactive shell starts in the workspace root (home dir when no
  project); full-screen TUI apps, colour and resize work.
• PTY spawn is the only OS-specific piece, behind the platform trait;
  session/emulation logic is shared, not duplicated.
• All PTY IO off the GTK main thread.
• Multiple terminal tabs; shell honours the Settings override then the
  OS default.
• Font from the editor monospace setting; colours from the src/css.rs
  palette in both themes.
• Open/closed state persisted with the rest of the layout.
• SCSS stem registered in css.rs + build.rs; icons via
  icons::img.
• cargo fmt + cargo clippy clean; syllable regression suite
  unaffected.