docs: add requirements-dev.txt install step before pytest in README

[R-J-N]

Summary

The README's "Verify Everything Works" section tells new users to run pytest tests/, but the Quick Start only installs requirements.txt, and pytest is in requirements-dev.txt. On a fresh setup, the documented test command fails because pytest isn't installed. This PR adds the missing install step and a changelog entry.

Related Issue

Closes #1943

Type of Change

• Documentation — updates docs, README, or code comments only

What Was Changed

File	Change made
README.md	In "Verify Everything Works", added pip install -r requirements-dev.txt before pytest tests/ and updated the sentence above it
CHANGELOG.md	Added an entry under "Fixed" for #1943

How to Test This PR

1. Check out this branch.
2. Create a fresh virtual environment and install: pip install -r requirements.txt
3. Run python -m pytest tests/. Before this change, this fails with "No module named pytest".
4. Run pip install -r requirements-dev.txt, then python -m pytest tests/ again. pytest now runs and collects 668 tests.

Test Results

Tested on macOS with Python 3.12 in a fresh virtual environment, after installing requirements-dev.txt:

28 failed, 637 passed, 4 skipped in 676.12s (0:11:16)

The 28 failures are in test_bookmarks.py, test_no_results_reset.py, test_og_tags.py, test_pagination.py, test_profile_sections.py, test_progress_calculation.py and test_share_project.py (missing OG/Twitter meta tags, progress-bar ARIA attributes, saved-projects panel, share toast, profile sections, and no project cards on /explore). This PR only changes docs and does not touch them.

Self-Review Checklist

• I have read CONTRIBUTING.md and followed all guidelines
• My branch name follows the convention: feat/, fix/, docs/, data/, style/, test/
• I have run pytest tests/ and all test suites pass
• I have run flake8 . locally and there are no errors
• I have not introduced any print() or console.log() debug statements
• Every new function I wrote has a docstring
• I have not modified files outside the scope of the linked issue
• If I changed the UI, I tested it at 375px (mobile) and 1280px (desktop)
• If I added a project to the dataset, it has all required JSON fields

Notes for Reviewer

• Two items are intentionally unticked. pytest tests/ does not fully pass on my clean setup (28 failures, listed above), and I didn't run flake8 . because no Python files changed. The last two items don't apply to a docs-only change.
• The README still says "660+ passed" as the expected output. I left it alone to keep this PR small. I can open a separate issue about the failing tests if that would help.
• Elsewhere the docs still mention python tests/test_basic.py (Contributing section) and "27 tests" (Project Structure). I can update those in this PR or a follow-up, whichever you prefer.

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