feat(terminal): add embedded PTY-backed terminal dock with per-tab sessions

Adds a built-in terminal dock that runs a real pseudo-terminal per repository tab.

- Backend: new src-tauri/src/terminal.rs implements TerminalState and Tauri commands
  terminal_open, terminal_write, terminal_resize and terminal_close using the
  portable-pty crate. Terminal output is emitted via "terminal:data" and exits
  via "terminal:exit". Includes unit tests for UTF-8 chunk handling. Cargo.toml
  updated to depend on portable-pty and main.rs registers the state/commands.
- Frontend: App.svelte/UI and app.css updated to show a resizable terminal dock,
  toggleable with Ctrl+` and persisted open/height state. TerminalPanel.svelte is
  lazy-loaded per repo tab (frontend provides session ids like repo:<path>).
  package.json adds @xterm/xterm and @xterm/addon-fit for the frontend terminal
  surface.
- Docs: CHANGELOG.md notes the new embedded terminal feature.

Behavior: each repository tab has its own PTY-backed shell started in the repo
cwd; the user's shell (SHELL/COMSPEC) is used so prompts, aliases and interactive
behavior work as in a normal terminal.

CHANGELOG.md

@@ -14,6 +14,10 @@ The project uses calendar-style versions in the form `YYYY.M.PATCH`.
   selection again.
 - Issues can be assigned to people, and labels are managed directly from the
   Issue Center.
+- An embedded terminal in the repository view, docked above the status bar and
+  adjustable in height. It runs a real pseudo terminal with the user's own
+  shell, so colors, interactive prompts, tab completion, and Ctrl+C work as
+  usual. Every repository tab keeps its own session; Ctrl + ` toggles the dock.
 
 ### Changed