feat(timeline): date certainty markers on world event cards

[CraigBuckmaster]

Parent epic: #1808
Depends on: #1810

Goal

Display the date_certainty field as a subtle marker on world event timeline cards.

Depends on

• Schema extension issue (adds certainty field to TimelineEntry)

Display rules

certainty	Year display
firm (default)	1754 BC (current behavior)
approximate	c. 1754 BC (preceded by "c.")
traditional	1754 BC followed by a small "trad." label in textMuted
disputed	1754 BC? (trailing "?") with the year text in italic

Only world events get the marker treatment. Biblical events have certainty=NULL and render as today.

File-by-file changes

MODIFY: app/src/components/timeline/TimelineEventCard.tsx

In the formatTimelineYear helper (or a new formatYearWithCertainty helper), branch on event.certainty for world events. Keep the existing formatTimelineYear for biblical events to avoid touching unrelated code.

MODIFY: app/__tests__/components/timeline/TimelineEventCard.test.tsx

Add tests for each certainty value's display.

Acceptance criteria

• Each of the four certainty values renders distinctly
• Biblical events (certainty=NULL) render unchanged
• All existing tests pass

Out of scope

• Tooltip / explanation modal for certainty markers (could come later)
• Filter or sort by certainty
• Accuracy auditor coverage

Branch / commit hygiene

• Branch: feat/timeline-certainty-markers
• Commit prefix: feat(timeline):