guard shell setup snippets against non-interactive logins

[ChrisJr404]

Closes #1551.

Why

The setup snippets we publish (and the ones the installer appends to user dotfiles) run fnm env unconditionally:

eval "$(fnm env --use-on-cd --shell bash)"

On Linux with a systemd user session, the display manager spawns a non-interactive login shell at session start to scrape env vars. With Plasma/SDDM the call tree is:

plasmalogin
 `-wayland-session
     `-fish --login -c /bin/sh -c 'export -p' > /tmp/xsess-env-...
         `- fnm env runs here, multishell is created, and systemd freezes it
            into the global session PATH

After that, every interactive terminal inherits the stale fnm_multishells/<dir> from the session env and adds its own on top. The visible symptom: fnm use system resolves to the older session-level multishell instead of /usr/bin/node. Detailed repro and pstree capture are in the linked issue.

Fix

Wrap the documented invocations and the installer-appended snippets in an interactive-shell check, so non-interactive parents leave PATH untouched:

bash and zsh

[[ $- == *i* ]] && eval "$(fnm env --use-on-cd --shell bash)"

fish

status is-interactive; and fnm env --use-on-cd --shell fish | source

Same check is added inside the existing if [ -d "$FNM_PATH" ] block in .ci/install.sh for bash, zsh and fish.

PowerShell and the WinCMD/Cmder snippets are left alone since the issue is specific to Linux display-manager session-init behavior, and adding an untested PowerShell host-interactive check has more downside than upside here.

Testing

Verified each guard locally against an actual non-interactive vs interactive invocation:

$ bash -c 'case $- in *i*) echo INTERACTIVE ;; *) echo NOT ;; esac'
NOT
$ bash -i -c 'case $- in *i*) echo INTERACTIVE ;; *) echo NOT ;; esac'
INTERACTIVE

$ zsh -c    '[[ $- == *i* ]] && echo INTERACTIVE || echo NOT'
NOT
$ zsh -i -c '[[ $- == *i* ]] && echo INTERACTIVE || echo NOT'
INTERACTIVE

$ fish    -c 'status is-interactive; and echo INTERACTIVE; or echo NOT'
NOT
$ fish -i -c 'status is-interactive; and echo INTERACTIVE; or echo NOT'
INTERACTIVE

bash -n, zsh -n and fish -n all parse the generated snippets cleanly.

.ci/test_installation_script.sh now runs the test shells with -i -c so the appended fnm block actually fires through the guard when sourcing the profile. Without that change the existing CI path (bash, zsh, fish all invoked as -c) would no-op the eval and the fnm install / fnm use steps would fail to find node on PATH.

A changeset (patch) is included.