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.

package.json

@@ -20,6 +20,8 @@
     "@tauri-apps/api": "^2.5.0",
     "@tauri-apps/plugin-dialog": "^2.7.1",
     "@tauri-apps/plugin-updater": "^2.10.1",
+    "@xterm/addon-fit": "^0.11.0",
+    "@xterm/xterm": "^6.0.0",
     "dompurify": "^3.4.15",
     "marked": "^18.0.12",
     "simple-icons": "^16.24.1",