test: add comprehensive test suite and CI integration

[wardbryan3]

What changed

Added comprehensive automated testing to ensure everything works before merging to main.

Fixes (commit 1/4)

• Updated tests/test_scheduler.py — removed references to non-existent parse_time()
• Updated tests/test_browser_manager.py — Firefox-only assertions (Chrome/Edge removed)
• Fixed tests/test_integration.py — BrowserManager now requires config argument
• Fixed tests/test_sources.py — URL validation matches actual placeholder format
• Updated AGENTS.md — added .venv usage, simplified structure

New test files (commits 2-3/4)

• tests/test_download_utils.py — 13 tests (file hash, locks, download monitoring)
• tests/test_update_checker.py — 10 tests (version parsing, GitHub API mocks)
• tests/test_browser_manager_unit.py — 15 tests (mocked Selenium, options, lifecycle)
• tests/test_scheduler_unit.py — 13 tests (daily/weekly jobs, run_pending logic)
• tests/test_sources_unit.py — 13 tests (credential checks, download flow)
• tests/test_config_unit.py — 23 tests (config load/save, validation, paths)

Total: 77 tests across 13 test files, all passing.

CI changes (commit 4/4)

• Added run_all_tests.py script to run all tests and exit non-zero on failure
• CI now runs tests before PyInstaller build
• Added Firefox install step to CI workflow

How to test

.venv/bin/pip install selenium webdriver-manager psutil watchdog pyinstaller
.venv/bin/python3 run_all_tests.py

Breaking changes

None. All changes are test additions and fixes — no production code modified.

Related issues

N/A