feat: optional --user, send/fetch config defaults, CodeQL fixes, v8.0.3

[KHolodilin]

Description

• password: --user optional for password set / password clear; fallback to imap.user from config.
• send folder: dir and --to optional; fallback to smtp.send_folder and smtp.default_recipient.
• send: python -m email_processor send (no subcommand) defaults to send folder with config.
• README: Quick Start (install, Fetch, Send, Send folder, Full pipeline); fetch/send use config by default; Features block removed.
• CodeQL: URL substring sanitization, workflow permissions, redact_email for logs.
• chore: Bump version to 8.0.3.

Closes #27

Type of Change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test addition or update
• Other (please describe):

Related Issue

Closes #27

Changes Made

• args: --user optional (password set/clear); dir/--to optional (send folder).
• main: user/folder/to from config when omitted; send without subcommand → send folder.
• README: Quick Start, Usage updates; fetch/send config defaults.
• CodeQL: test_client substring fix, workflow permissions, redact_email in auth/smtp/fetcher.
• Version: 8.0.3 in __version__.py and pyproject.toml.

Testing

• All existing tests pass (731 passed, 32 skipped)
• New tests added for new functionality
• Manual testing performed (if applicable)
• Test coverage maintained or improved

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published
• Pre-commit hooks pass (pre-commit run --all-files)
• Code formatting passes (ruff format --check .)
• Linting passes (ruff check .)

Additional Notes

• fetch and send use config by default.
• send = send from folder (config).

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