Add fzf-tab plugin (navigable Tab completion menu)

Kaiser's existing .zshrc had fzf-tab added to the plugins list —
it gives a navigable Tab completion menu with descriptions for each
match. Adding to the canonical plugin list so kaiser doesn't lose
functionality when the repo version overwrites the box-specific
one. Strictly additive: doesn't affect miche or other boxes.

Adds:
- fzf-tab to dot_zshrc.tmpl plugins array
- install_zsh_plugin Aloxaf/fzf-tab to run_once_20

dot_zshrc.tmpl

@@ -22,6 +22,7 @@ plugins=(
     rust
     rsync
     command-not-found
+    fzf-tab                       # navigable Tab completion menu
     zsh-syntax-highlighting       # fish-style live colors — MUST be near last
     zsh-history-substring-search  # fish-style Up/Down history search
     zsh-autosuggestions           # fish-style ghost completions — MUST be last