feat: Notion backend for the catalog + set command + ls/source/save UX

Persist the entry catalog to a Notion page instead of an external script.

- `hel store notion:<ref>` reads the dump from stdin and writes it into a
  single code block on the page; `hel load notion:<ref>` prints it back.
  `<ref>` is a page id (UUID) or a page title (case-insensitive search).
  Token from HEL_NOTION_TOKEN. Networking lives in helcli (ureq + serde_json);
  the core lib and the wasm build are untouched. argv is dispatched before the
  REPL starts, so the subcommands never read ~/.helrc or prompt for a master.

- `set <key> <value>`: runtime config (e.g. hel_dump, hel_notion_token) from
  ~/.helrc instead of env vars. The map is exported (uppercased) into spawned
  pipe/source children, so `set hel_notion_token ...` reaches `hel store`.
  Values are redacted in history.

- `ls` now matches case-insensitively.
- `source` echoes `source <target>`.
- `save` prints a `< removed` / `> added` diff against the last load/save
  snapshot so removals are noticed before they persist.

hel/src/utils.rs

@@ -206,6 +206,9 @@ pub mod editor {
 pub fn call_cmd_with_input(cmd: &str, args: &Vec<String>, input: &str) -> io::Result<String> {
     let mut cmd = Command::new(cmd)
         .args(args)
+        // Export runtime `set …` config (uppercased) into the child, so e.g.
+        // `set hel_notion_token …` reaches a spawned `hel store`/`hel load`.
+        .envs(crate::structs::config_envs())
         .stdin(Stdio::piped())
         .stdout(Stdio::piped())
         .spawn()?;