feat(cli): completions, library/audio commands, server switcher

Embed bash and zsh completion scripts and add a `completions` subcommand.
Extend the player CLI (library, audio device, instant mix) and surface
`music_library` in snapshots. Add a shared active-server switcher in the
header and locales; instant mix can reseed the queue from CLI and UI.

completions/psysonic.bash

@@ -0,0 +1,111 @@
+# bash completion for Psysonic (see `psysonic --help`).
+# Install: source /path/to/completions/psysonic.bash
+# Optional: jq + prior `psysonic --player audio-device list` for device name completion.
+#
+# Uses no `mapfile` so bash 3.2 (macOS default) works.
+
+_psysonic_compreply_from_compgen() {
+  # $1 = compgen -W word list, $2 = current word
+  COMPREPLY=()
+  local line
+  while IFS= read -r line; do
+    [[ -n $line ]] && COMPREPLY+=("$line")
+  done < <(compgen -W "$1" -- "$2")
+}
+
+_psysonic_audio_device_json() {
+  local f
+  if [[ -n ${XDG_RUNTIME_DIR:-} ]]; then
+    f="$XDG_RUNTIME_DIR/psysonic-cli-audio-devices.json"
+    [[ -r $f ]] && { printf '%s' "$f"; return; }
+  fi
+  f="${TMPDIR:-/tmp}/psysonic-cli-audio-devices.json"
+  [[ -r $f ]] && printf '%s' "$f"
+}
+
+_psysonic_library_json() {
+  local f
+  if [[ -n ${XDG_RUNTIME_DIR:-} ]]; then
+    f="$XDG_RUNTIME_DIR/psysonic-cli-library.json"
+    [[ -r $f ]] && { printf '%s' "$f"; return; }
+  fi
+  f="${TMPDIR:-/tmp}/psysonic-cli-library.json"
+  [[ -r $f ]] && printf '%s' "$f"
+}
+
+_psysonic_complete() {
+  local cur
+  cur="${COMP_WORDS[COMP_CWORD]}"
+
+  local i pidx=0
+  for (( i = 1; i < COMP_CWORD; i++ )); do
+    [[ ${COMP_WORDS[i]} == --player ]] && pidx=$i
+  done
+
+  if (( pidx == 0 )); then
+    if [[ ${COMP_WORDS[1]} == completions && COMP_CWORD -eq 2 ]]; then
+      _psysonic_compreply_from_compgen 'help bash zsh' "$cur"
+      return
+    fi
+    _psysonic_compreply_from_compgen '--help --version --info --json --quiet --player completions' "$cur"
+    return
+  fi
+
+  local -a sub=()
+  for (( i = pidx + 1; i < COMP_CWORD; i++ )); do
+    sub+=("${COMP_WORDS[i]}")
+  done
+  local n=${#sub[@]}
+
+  if (( n == 0 )); then
+    _psysonic_compreply_from_compgen 'next prev play pause seek volume audio-device library mix' "$cur"
+    return
+  fi
+
+  case ${sub[0]} in
+    audio-device)
+      if (( n == 1 )); then
+        _psysonic_compreply_from_compgen 'list set' "$cur"
+      elif [[ ${sub[1]} == set ]] && (( n == 2 )); then
+        COMPREPLY=()
+        local jf d
+        jf="$(_psysonic_audio_device_json)"
+        if [[ -n $jf ]] && command -v jq &>/dev/null; then
+          while IFS= read -r d; do
+            [[ -n $d && $d == "$cur"* ]] && COMPREPLY+=("$d")
+          done < <(jq -r '.devices[]? | select(type == "string")' "$jf" 2>/dev/null)
+        fi
+        while IFS= read -r line; do
+          [[ -n $line ]] && COMPREPLY+=("$line")
+        done < <(compgen -W 'default' -- "$cur")
+        ((${#COMPREPLY[@]})) && compopt -o filenames 2>/dev/null
+      fi
+      ;;
+    library)
+      if (( n == 1 )); then
+        _psysonic_compreply_from_compgen 'list set' "$cur"
+      elif [[ ${sub[1]} == set ]] && (( n == 2 )); then
+        COMPREPLY=()
+        local jf id line
+        jf="$(_psysonic_library_json)"
+        if [[ -n $jf ]] && command -v jq &>/dev/null; then
+          while IFS= read -r id; do
+            [[ -n $id && $id == "$cur"* ]] && COMPREPLY+=("$id")
+          done < <(jq -r '.folders[]? | select(.id != null) | .id | tostring' "$jf" 2>/dev/null)
+        fi
+        while IFS= read -r line; do
+          [[ -n $line ]] && COMPREPLY+=("$line")
+        done < <(compgen -W 'all' -- "$cur")
+        ((${#COMPREPLY[@]})) && compopt -o filenames 2>/dev/null
+      fi
+      ;;
+    mix)
+      (( n == 1 )) && _psysonic_compreply_from_compgen 'append new' "$cur"
+      ;;
+    seek|volume)
+      (( n == 1 )) && compopt -o default && COMPREPLY=()
+      ;;
+  esac
+}
+
+complete -F _psysonic_complete psysonic