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/repl.rs

@@ -130,6 +130,7 @@ impl<'a> LKEval<'a> {
                 quit = self.cmd_source(&out, script);
             }
             Command::Dump(script) => self.cmd_dump(&out, script),
+            Command::Set(key, value) => { to_history = false; self.cmd_set(&out, key, value); }
             Command::Pass(name, None) => self.cmd_pass(&out, &name, &None),
             Command::Pass(name, pass) => { to_history = false; self.cmd_pass(&out, &name, &pass); },
             Command::UnPass(name) => match self.state.lock().borrow_mut().secrets.remove(name) {