Warn when scheduled WattPlan tasks are disabled

[LordMike]

Summary

• Add a targeted options-flow acknowledgement before saving scheduler settings that disable scheduled planning and/or scheduled action emission.
• Preserve staged disabled timer values when the warning is declined so users return to Scheduler settings with their previous choices still suggested.
• Document manual service responsibility when automatic scheduler stages are disabled.

Closes #35.

This was detected from review of #29.

Tests

• WATTPLAN_TEST_VENV=/tmp/wattplan-venv-shared ./scripts/run_tests.sh tests/integration/test_config_flow.py -q
• WATTPLAN_TEST_VENV=/tmp/wattplan-venv-shared ./scripts/run_tests.sh
• python -m json.tool custom_components/wattplan/strings.json
• python -m json.tool custom_components/wattplan/translations/en.json
• git diff --check

Screenshots

Skipped. I attempted to boot Home Assistant from /tmp/wattplan-issue35-ha-config; the first run hit an occupied 8123 port and missing default_config dependencies in the shared test venv. A minimal config did boot on port 8135, but completing frontend onboarding/auth and creating enough UI state for options-flow screenshots was too brittle for the stretch goal.