feat: Added KPI variation pills

[ad4mou]

Summary

• KPI cards can now show a small change pill under the number, comparing the latest period to the one before it.
• Three styles: percentage (+20%, green/red with arrow), variation (signed change, green/red with arrow), and absolute (just the magnitude, no color/arrow) - or you can hide it.
• The pill names the period it compares against -> "vs. last month / last week / last quarter / last year", inferred from the data's dates (falls back to "vs. previous period" when spacing is unclear; handles month-only and datetime values).
• Style is switchable from the chart's edit dialog, shown only when a variation pill can actually be added.
• The agent is guided to return a metric across consecutive periods and turn the pill on when a period-over-period change is worth showing.

Fixes

• KPI cards with no x_axis_key were silently dropped when parsing stories (blank cards) -> now parsed correctly.
• The edit pencil button no longer floats above KPI cards; it sits in the card's top-right corner.

Closes #1180

[github-actions]

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[cubic-dev-ai]

All reported issues were addressed across 13 files

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