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.

helcli/Cargo.toml

@@ -12,3 +12,5 @@ name = "hel"
 [dependencies]
 hel = { version = "0.1.0", path = "../hel" }
 rustyline = "10.0.0"
+ureq = { version = "2", features = ["json"] }   # blocking HTTP, rustls default — no openssl
+serde_json = "1"