feat(submit): add solution submission UI

Add submit.lua that reads credentials from a local JSON store (prompting
via vim.ui.input/inputsecret on first use), reads the source file, and
delegates to scraper.submit(). Add language_ids.py with platform-to-
language-ID mappings for atcoder, codeforces, and cses.

scrapers/language_ids.py

@@ -0,0 +1,18 @@
+LANGUAGE_IDS = {
+    "atcoder": {
+        "cpp": "5028",
+        "python": "5078",
+    },
+    "codeforces": {
+        "cpp": "89",
+        "python": "70",
+    },
+    "cses": {
+        "cpp": "C++17",
+        "python": "Python3",
+    },
+}
+
+
+def get_language_id(platform: str, language: str) -> str | None:
+    return LANGUAGE_IDS.get(platform, {}).get(language)