Fix change-start-date resolution

[v-aidaba]

Fix change-start-date, add landing page and empty states

Start date ("Change start date"):

• Match the selected start date by calendar day (fixes time/timezone off-by-one)
• Missing day (gap) falls back to the next available date
• Out-of-range date falls back to the closest available date instead of a 0% change
• Invalid text input is ignored and flagged, not silently applied
• Warning hint icon explains any adjustment (gap / out-of-range / invalid)
• New "Show start date on chart" KPI toggle to display "(since )" by the days count

Landing page & empty states:

• Add landing page with the visual icon (supportsLandingPage / supportsEmptyDataView)
• Empty-state messages for missing Date/Values, invalid/blank dates, and no valid data