feat: make --user optional in password set/clear, use imap.user from config

[KHolodilin]

Description

Make --user optional for password set and password clear. When --user is omitted, use imap.user from the configuration file (config.yaml).

Current behavior:

# Required
python -m email_processor password set --user your_email@example.com
python -m email_processor password clear --user your_email@example.com

Desired behavior:

# With config containing imap.user
python -m email_processor password set
python -m email_processor password clear

# Explicit --user still overrides
python -m email_processor password set --user other@example.com

Tasks

• Make --user optional in CLI args for password set and password clear (required=False)
• When --user is not provided, load config (--config path) and use imap.user
• If neither --user nor imap.user is available, error: e.g. --user is required or set imap.user in config
• Update __main__ flow: ensure config is loaded before password commands when resolving user
• Update tests (unit + integration) for both --user and config fallback
• Update README / Quick Start if password set examples assume --user is mandatory

Acceptance Criteria

• password set and password clear work without --user when imap.user is in config
• --user overrides imap.user when both are present
• Clear error when neither is available
• All tests pass, pre-commit passes
• Documentation updated

Notes

• Config is already loaded for commands that use it; password commands run in a code path where config can be loaded (or we load it only when --user is missing to avoid breaking password clear without config).
• Consider same fallback for password clear for consistency.

Implementation: PR #28

[KHolodilin]

README Quick Start обновлён в PR #28: установка модуля (pip install email-processor, без .venv), First Run разбит на Fetch и Send, блок Features & Improvements удалён.

[KHolodilin]

В PR #28 дополнительно:

• send folder: dir и --to опциональны, fallback на smtp.send_folder / smtp.default_recipient.
• send без подкоманды: python -m email_processor send = send folder с config.
• README: fetch и send используют config по умолчанию; Quick Start обновлён (шаги 5–8).