[M8] Frontend: note signal evidence block in insight cards

[Sturmi77]

Context

Part of the Notes in Analysis Epic. This issue adds the visual evidence block to insight cards, making note-derived signals visible and transparent for the user.

Depends on: #201 (signal extraction), #198 (marker-aware insights)

Spec Reference

docs/features/notes-in-analysis.md — Section: Insights — Evidence Block

Evidence Block Design

┌──────────────────────────────────────────────────┐
│ 💡 An Tagen mit Marker "Stress" lag dein Mood    │
│    im Schnitt 1.4 Punkte unter deinem Mittelwert │
│                                                  │
│    Basis: 14 Tage · Konfidenz: 0.82              │
│    Beispiele: 12. März, 3. April, 9. Mai         │
└──────────────────────────────────────────────────┘

Tasks

• Render evidence block on insight cards when sample_size >= 20
• Show: signal/marker name, avg_delta, sample_size, time_window, confidence, up to 3 example dates
• Soft prompt when sample_size < 20: “Füge Notizen hinzu, um Zusammenhänge besser zu verstehen.”
• Insight card hidden (not error) when below threshold
• Confidence displayed as percentage or label (e.g. “Mittel” / “Hoch”), not raw decimal
• Example dates are clickable and open the entry drawer from [Retroactive M2] Frontend: note visibility in timeline, calendar, and analysis drilldown #196
• Mobile layout tested at 375px
• No medical/causal copy — copy review required before merge

Acceptance Criteria

• Evidence block rendered correctly at sample_size >= 20
• Below-threshold prompt shown and styled correctly
• Example date links open correct entry drawer
• Copy reviewed — no causal or diagnostic language
• Mobile layout correct at 375px