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.

lua/cp/submit.lua

@@ -0,0 +1,106 @@
+local M = {}
+
+local logger = require('cp.log')
+local state = require('cp.state')
+
+local credentials_file = vim.fn.stdpath('data') .. '/cp-nvim-credentials.json'
+
+local function load_credentials(platform)
+  if vim.fn.filereadable(credentials_file) ~= 1 then
+    return nil
+  end
+  local content = vim.fn.readfile(credentials_file)
+  if #content == 0 then
+    return nil
+  end
+  local ok, data = pcall(vim.json.decode, table.concat(content, '\n'))
+  if not ok then
+    return nil
+  end
+  return data[platform]
+end
+
+local function save_credentials(platform, creds)
+  local data = {}
+  if vim.fn.filereadable(credentials_file) == 1 then
+    local content = vim.fn.readfile(credentials_file)
+    if #content > 0 then
+      local ok, decoded = pcall(vim.json.decode, table.concat(content, '\n'))
+      if ok then
+        data = decoded
+      end
+    end
+  end
+  data[platform] = creds
+  vim.fn.mkdir(vim.fn.fnamemodify(credentials_file, ':h'), 'p')
+  vim.fn.writefile({ vim.json.encode(data) }, credentials_file)
+end
+
+local function prompt_credentials(platform, callback)
+  local saved = load_credentials(platform)
+  if saved and saved.username and saved.password then
+    callback(saved)
+    return
+  end
+  vim.ui.input({ prompt = platform .. ' username: ' }, function(username)
+    if not username or username == '' then
+      logger.log('Submit cancelled', vim.log.levels.WARN)
+      return
+    end
+    vim.fn.inputsave()
+    local password = vim.fn.inputsecret(platform .. ' password: ')
+    vim.fn.inputrestore()
+    if not password or password == '' then
+      logger.log('Submit cancelled', vim.log.levels.WARN)
+      return
+    end
+    local creds = { username = username, password = password }
+    save_credentials(platform, creds)
+    callback(creds)
+  end)
+end
+
+function M.submit(opts)
+  local platform = state.get_platform()
+  local contest_id = state.get_contest_id()
+  local problem_id = state.get_problem_id()
+  local language = (opts and opts.language) or state.get_language()
+  if not platform or not contest_id or not problem_id or not language then
+    logger.log('No active problem. Use :CP <platform> <contest> first.', vim.log.levels.ERROR)
+    return
+  end
+
+  local source_file = state.get_source_file()
+  if not source_file or vim.fn.filereadable(source_file) ~= 1 then
+    logger.log('Source file not found', vim.log.levels.ERROR)
+    return
+  end
+
+  prompt_credentials(platform, function(creds)
+    local source_lines = vim.fn.readfile(source_file)
+    local source_code = table.concat(source_lines, '\n')
+
+    require('cp.scraper').submit(
+      platform,
+      contest_id,
+      problem_id,
+      language,
+      source_code,
+      creds,
+      function(result)
+        vim.schedule(function()
+          if result and result.success then
+            logger.log('Submitted successfully', vim.log.levels.INFO, true)
+          else
+            logger.log(
+              'Submit failed: ' .. (result and result.error or 'unknown error'),
+              vim.log.levels.ERROR
+            )
+          end
+        end)
+      end
+    )
+  end)
+end
+
+return M

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)