Add fullscreen toggle and fix shell integration resources

[elmarco]

Adds a configurable F11 fullscreen shortcut and a Toggle Fullscreen command-palette entry. Client-side headers hide in fullscreen and return on exit, including when decoration mode changes while fullscreen.

Also fixes shell startup and resource placement:

• Copy Ghostty's .zshenv into the runtime wrapper directory. A symlink resolves back to Ghostty's original directory and bypasses Séance's integration; the copy loads user startup files and both sets of hooks.
• Install and locate Séance's shell scripts and executable wrappers under share/seance.

CI installs Openbox and zsh for the GUI and shell suites. It removes the runner’s unused Chrome APT source before updating package indexes, avoiding an upstream checksum failure that blocked dependency installation.

Validation:

• 167 Zig unit tests and 24 E2E tests passed.
• 27 Python tests passed, covering fullscreen, Bash/zsh startup, Ghostty embedding, keyboard remaps, IME, and Codex wrappers. The optional live Codex test was skipped.
• Fullscreen tests use physical keys, GTK, and Openbox on isolated X11 displays. They cover F11, the command palette, rebinding, decoration changes, and terminal input. Wayland was not exercised.
• Regression checks fail for F11 on the previous main and for Séance's zsh hooks with the proposed symlink. A clean installation passes all three shell tests.
• Shell tests also pass on Ubuntu 24.04. Their disposable profiles skip distro-global completion initialization, which otherwise blocks on an interactive permissions prompt on the CI image. Startup errors include terminal output in the failure report.
• The maintainer tested F11 locally and reran all three shell tests successfully.

[no1msd]

Thanks @elmarco! Fixed on main in aa7d18c, preserving your authorship in one commit.

CI passed on the final commit, including all three fullscreen tests, all three Bash/zsh integration tests, and the existing suites. Local validation also passed 167 unit tests, 24 E2E tests, and 27 Python tests; the maintainer tested F11 and confirmed all three shell tests. Automated GUI coverage used X11, and the optional live Codex test was skipped.

The zsh bootstrap is copied so both Ghostty and Séance hooks load. Shell resources now live under share/seance, and fullscreen keeps the client-side header hidden across decoration changes.