feat: cross-platform installer with JSON-mode integration

Adds install.bat / install.sh / install.mjs at the project root for a
one-shot setup: prereqs check, renderer clone & link, dependency install,
config copy, JSON parser mode selection, URL prompt with validation, and
the production build.

- install.bat / install.sh: thin OS-specific wrappers around install.mjs
- install.mjs: 9-step installer with --help, --non-interactive,
  --skip-build/clone/link and per-URL override flags
- new step 6 'Choose JSON parsing mode': prompts the operator (json5
  recommended) or accepts --json-mode=json5|legacy|auto in CI; writes
  .nitro-build.json so the final 'yarn build' picks it up directly
- summary now reports the selected JSON mode and its source
- .gitattributes: force LF on install.sh / install.mjs so the shebang
  stays valid on Linux/macOS after a Windows checkout
- install.sh marked executable in the index (100755)
- README: new 'Quick install' section with interactive and CI usage,
  plus a complete --non-interactive example

.gitattributes

@@ -0,0 +1,4 @@
+install.sh   text eol=lf
+install.mjs  text eol=lf
+*.sh         text eol=lf
+*.bat        text eol=crlf