feat(experiments): add configurable baseline variant

[lordspline]

This PR introduces configurable baseline variants for experiments, allowing users to select any variant as the baseline instead of assuming control. The change is backward compatible and updates calculations, validation, and UI labels across the experiment stack.

Backend

• products/experiments/backend/hogql_queries/experiment_trends_query_runner.py: Read baseline_variant_key from stats_config and use it to determine baseline instead of hardcoded CONTROL_VARIANT_KEY
• products/experiments/backend/hogql_queries/experiment_funnels_query_runner.py: Same baseline configuration logic for funnel metrics
• products/experiments/backend/hogql_queries/utils.py: Updated error messages to reference the configured baseline key
• products/experiments/backend/hogql_queries/test/test_stats_config.py: Added tests for legacy query runners respecting baseline_variant_key

Frontend

• frontend/src/scenes/experiments/MetricsView/shared/ErrorChecklist.tsx: Dynamic error messages using baseline_variant_key from stats_config
• frontend/src/scenes/experiments/legacy/metricsView/LegacyErrorChecklist.tsx: Same baseline-aware error messaging
• frontend/src/scenes/experiments/constants.ts: Updated conclusion descriptions to use "baseline" instead of "control"
• frontend/src/scenes/experiments/stories/ExperimentFrequentistFiveVariants.stories.tsx: Renamed story to ExperimentFrequentistFiveVariantsCustomBaseline to demonstrate the feature

Fixtures

• frontend/src/mocks/fixtures/api/experiments/experiment_frequentist_five_variants.json: Added baseline_variant_key: "test-2" to stats_config
• frontend/src/mocks/fixtures/api/experiments/experiment_frequentist_five_variants_*_metric_result.json: Updated mock results to reflect test-2 as the baseline with other variants as test variants

Existing experiments remain compatible because baseline_variant_key defaults to control when not set.

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[capy-ai]

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