fix(app): smooth history chart joins

[frahlg]

What changed

• draw history traces with shape-preserving cubic curves
• keep every measured point, extrema, data gap, and finger readout intact
• apply the same bounded smoothing to reduced long-range mean lines
• add regression coverage and a patch changeset

Why

The 24-hour history view joins 288 five-minute readings with straight segments. Frequent small direction changes therefore appeared as sharp corners on a phone.

User impact

History traces look smoother while real steps and spikes remain visible. The chart does not interpolate across missing data or overshoot measured values.

Validation

• npm run verify
• 864 tests passed; 1 skipped
• Svelte check: 0 errors and 0 warnings
• production Vite build
• before/after visual comparison at 390 px mobile width against the same deterministic simulated home

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	ftw-webapp	2acb7c3	Commit Preview URL Branch Preview URL	Aug 12 2026, 09:43 AM