Concept: add fish shell and zellij session backend defaults

[Pyrobrick]

Concept

This is a concept implementation for making the add-on more beginner-friendly and less tmux/ZSH-opinionated by introducing selectable shell and session backend options.

What changed

• Adds a new shell option with fish, zsh, and bash choices.
• Defaults new installations to shell: fish.
• Keeps the old zsh boolean as a legacy fallback for existing configurations that do not yet define shell.
• Adds a new session_backend option with zellij and tmux choices.
• Defaults new installations to session_backend: zellij.
• Installs Fish and Zellij in the image.
• Generates Fish history persistence and Fish Supervisor-token setup.
• Updates the Web Terminal startup logic to use the selected shell/backend.
• Updates SSH login handoff so the configured non-root SSH user uses a neutral /bin/sh wrapper and hands off to the selected root login shell via sudo -i.

Why

Fish has much better beginner ergonomics for interactive CLI use: helpful completions, autosuggestions, syntax highlighting, and generally fewer papercuts for people who only occasionally need a terminal.

Zellij is a more discoverable terminal workspace than tmux for casual Home Assistant users. It should make shared sessions and terminal panes easier to understand without expecting users to know tmux keybindings.

Notes / caveats

This is intentionally a concept PR, not a polished upstream-ready patch yet.

Things to verify before treating this as production-ready:

• Whether fish and zellij are available in the target Alpine repositories for both supported architectures.
• Whether the unpinned fish and zellij package additions should be pinned to match the current Dockerfile style.
• Whether zellij attach --create homeassistant options --default-shell <shell> behaves exactly as intended in the ttyd ingress environment.
• Whether shared SSH/Web Terminal sessions behave correctly for all combinations:
  • shell: fish, session_backend: zellij
  • shell: fish, session_backend: tmux
  • shell: zsh, session_backend: zellij
  • shell: bash, session_backend: tmux
• Whether the legacy zsh option should eventually be deprecated in documentation or removed in a future breaking release.

In short: this is the UX direction I would want to test first, not a claim that the exact implementation is already perfect.