fix: keep KNOWN_PROVIDERS in sync with PROVIDERS registry (#53)

[jayrav13]

Summary

Closes #53.

The "See it in action" GIF on https://jayravaliya.com/tenor-tui/ was rendering as a broken shell-error video because of a config-validation bug, not (just) a stale-asset issue.

Root cause: --provider fixture looked valid (argparse reads choices from PROVIDERS.keys()), but load_config() validated the resolved provider against a hardcoded KNOWN_PROVIDERS = {"yahoo", "tradier"} that had drifted when FixtureProvider was added. So tenortui --provider fixture exited with Configuration error: Unknown provider 'fixture', and that error is exactly what VHS captured into the demo GIFs.

Fix:

• src/tenortui/config.py — derive KNOWN_PROVIDERS from PROVIDERS.keys() so the registry is the single source of truth.
• tests/test_config.py — add a parametrized regression test that asserts every registered provider is accepted by load_config(provider_override=...), plus a guard that KNOWN_PROVIDERS matches PROVIDERS.keys().
• tests/test_settings_screen.py — provider-cycle test now computes the expected next provider from KNOWN_PROVIDERS instead of hardcoding "tradier" (which was correct only when fixture was excluded).
• docs/tapes/*.tape — added a comment to each tape documenting why --provider fixture is the right choice for recordings.
• docs/site/docs/assets/demos/*.gif — all three demo GIFs re-recorded against the fixed code; they now show the working TUI instead of shell errors.
• CHANGELOG.md — entry under [Unreleased] / Fixed.

Test plan

• poetry run python -m pytest — 434 passed
• poetry run ruff check src/ tests/ — clean
• poetry run ruff format --check src/ tests/ — 56 files formatted
• make docs-strict — builds clean
• make demos — all 3 GIFs regenerated; spot-checked frames show TUI options chain rendering with fixture provider (not the previous shell-error capture)
• Pre-commit + pre-push hooks both green
• CI green on this PR

Notes / follow-ups

• Settings-screen "default provider" cycle now includes fixture because it sorts via KNOWN_PROVIDERS. That's a minor UX wart (an end user pressing Enter on the default-provider row could land on the demo provider). Hiding internal-only providers from user-facing pickers is a reasonable separate ticket — kept it out of this PR to keep the diff focused on the bug.

Co-authored by Claude