FEAT: Plugin API - account info API

[andrinoff]

Is your feature request related to a problem?

Plugins have no way to query account information. There is no matcha.accounts() or matcha.current_account() API. Multi-account setups can't have account-specific plugin behavior (e.g. different signatures per account, different auto-BCC rules, account-specific workflows).

Describe the solution you'd like

Add API functions:

-- Get the currently active account
local acct = matcha.current_account()
-- acct.email, acct.name, acct.id

-- List all configured accounts
local accounts = matcha.accounts()
for _, acct in ipairs(accounts) do
    matcha.log(acct.email)
end

Only expose safe, read-only fields (email, display name, account ID). Do not expose passwords, tokens, or server credentials.

Describe alternatives you've considered

Using the account_id field already present in hook email tables and hardcoding known IDs. This is fragile and doesn't let plugins discover accounts dynamically.

Additional context

N/A