chore(pack): add CPack configuration to build a deb package

Add install rules and CPack settings so cpack produces an installable
.deb. Dependencies are derived with dpkg-shlibdeps, so they follow the
binaries. The sherpa-onnx runtime the streaming backend needs is bundled
because no distro package provides it. The fetch-models script now also
locates its registry when installed system-wide.

增加 CPack 配置，可直接用 cpack 打出 deb：安装规则含桌面入口、应用图标、
模型注册表与下载脚本、用户文档；依赖由 dpkg-shlibdeps 自动推导；流式后端
需要的 sherpa-onnx 运行时（发行版无对应包）随包携带；fetch-models 脚本
装到系统后也能找到注册表。

scripts/fetch-models.sh

@@ -10,11 +10,26 @@
 #   ${XDG_DATA_HOME:-~/.local/share}/live-caption/models/<id>/
 set -euo pipefail
 
-ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
-REG="$ROOT/data/models.json"
+SELF="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+# 注册表位置：既支持在源码树里跑（<repo>/scripts/ 与 <repo>/data/），
+# 也支持装到系统之后跑（/usr/bin/ 与 /usr/share/live-caption/）。
+REG="${LIVE_CAPTION_REGISTRY:-}"
+if [ -z "$REG" ]; then
+  for cand in "$SELF/../data/models.json" \
+              "$SELF/../share/live-caption/models.json" \
+              "/usr/share/live-caption/models.json" \
+              "/usr/local/share/live-caption/models.json"; do
+    if [ -f "$cand" ]; then REG="$(cd "$(dirname "$cand")" && pwd)/$(basename "$cand")"; break; fi
+  done
+fi
 DEST="${XDG_DATA_HOME:-$HOME/.local/share}/live-caption/models"
 
-[ -f "$REG" ] || { echo "找不到注册表: $REG" >&2; exit 1; }
+[ -n "$REG" ] && [ -f "$REG" ] || {
+  echo "找不到模型注册表（models.json）。" >&2
+  echo "可用 LIVE_CAPTION_REGISTRY=<path> 指定。" >&2
+  exit 1
+}
 
 ids=$(python3 -c "
 import json,sys