feat(dashboard): rename 7-day-average to 7-day-trend with delta indicator per metric

The dashboard tiles labelled the 7-day average as "7T:" / "7d:" with
the mean value beside it. Marc found that label unclear when the
goal of the tile is to convey *direction* over the past week, not a
mid-point average:

  Before:  7T:    80,5     30T:    79,8
  After:   7T-Trend:  80,5 (+1.2)   30T:    79,8

The avg7 line now flips its label to `charts.trend7dShort` ("7d trend"
/ "7T-Trend") when a `trend7Delta` prop is supplied, and renders a
signed delta indicator next to the average. Color of the delta
follows the same `directionSentiment` rules the headline arrow
already uses:

- up-bad metrics (BP, weight, body fat): +delta paints orange,
  -delta paints green.
- up-good metrics (mood, sleep, steps): +delta paints green,
  -delta paints orange.
- neutral metrics (pulse): both signs stay muted.

Tiny absolute deltas (< 0.05) read as "no movement" and stay muted
regardless of sentiment, so a flat metric never paints a colour.

Delta source is the existing 7-day linear-regression slope from
`DataSummary.slope7` (already computed by `summarize()` in
`lib/analytics/trends.ts`). Projection: `slope.slope * 7 days =
end-to-end delta of the regression line` — extracted into the pure
helper `summaryToTrend7Delta()` in `lib/analytics/trend-delta.ts` so
the rule stays unit-testable.

Wired to weight, BP-sys, BP-dia, pulse, body fat, mood, sleep, steps.
Glucose tiles keep the legacy avg label until v1.5 (per-context
slopes need a wider summary contract).

Tests:

- Unit `summaryToTrend7Delta()`: null when summary missing /
  slope7 null, projects rate × 7 for positive and negative slopes,
  zero for stable slope.
- Component `<TrendCard>`: trend label ("7d trend" / "7T-Trend")
  shows in EN + DE, legacy label preserved when no delta supplied
  (back-compat), delta renders with `+`/`−` sign and one decimal,
  ±0 for near-zero, +delta on up-bad → orange, +delta on up-good →
  green, neutral metric → muted regardless of sign.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/page.tsx

@@ -40,6 +40,7 @@ import { MeasurementForm } from "@/components/measurements/measurement-form";
 import { MoodForm } from "@/components/mood/mood-form";
 import { TrendCard } from "@/components/charts/trend-card";
 import { TrendHint } from "@/components/charts/trend-hint";
+import { summaryToTrend7Delta } from "@/lib/analytics/trend-delta";
 import { GettingStartedChecklist } from "@/components/onboarding/getting-started-checklist";
 
 const HealthChart = dynamic(
@@ -471,6 +472,7 @@ export default function DashboardPage() {
                   fmt.number,
                 )}
                 slope30={w?.slope30 ?? null}
+                trend7Delta={summaryToTrend7Delta(w)}
                 icon={Activity}
                 directionSentiment="up-bad"
               />
@@ -513,6 +515,7 @@ export default function DashboardPage() {
                   fmt.number,
                 )}
                 slope30={sys?.slope30 ?? null}
+                trend7Delta={summaryToTrend7Delta(sys)}
                 icon={Heart}
                 directionSentiment="up-bad"
               />
@@ -550,6 +553,7 @@ export default function DashboardPage() {
                   fmt.number,
                 )}
                 slope30={dia?.slope30 ?? null}
+                trend7Delta={summaryToTrend7Delta(dia)}
                 icon={Heart}
                 directionSentiment="up-bad"
               />
@@ -587,6 +591,7 @@ export default function DashboardPage() {
                   fmt.number,
                 )}
                 slope30={p?.slope30 ?? null}
+                trend7Delta={summaryToTrend7Delta(p)}
                 icon={TrendingUp}
               />
             ),
@@ -605,6 +610,7 @@ export default function DashboardPage() {
                 avg7={bf?.avg7 ?? null}
                 avg30={bf?.avg30 ?? null}
                 slope30={bf?.slope30 ?? null}
+                trend7Delta={summaryToTrend7Delta(bf)}
                 icon={Percent}
                 directionSentiment="up-bad"
               />
@@ -624,6 +630,7 @@ export default function DashboardPage() {
                 avg7={moodSummary?.avg7 ?? null}
                 avg30={moodSummary?.avg30 ?? null}
                 slope30={moodSummary?.slope30 ?? null}
+                trend7Delta={summaryToTrend7Delta(moodSummary)}
                 icon={Smile}
                 directionSentiment="up-good"
               />
@@ -643,6 +650,7 @@ export default function DashboardPage() {
                 avg7={sleepSummary?.avg7 ?? null}
                 avg30={sleepSummary?.avg30 ?? null}
                 slope30={sleepSummary?.slope30 ?? null}
+                trend7Delta={summaryToTrend7Delta(sleepSummary)}
                 icon={Moon}
                 directionSentiment="up-good"
               />
@@ -662,6 +670,7 @@ export default function DashboardPage() {
                 avg7={stepsSummary?.avg7 ?? null}
                 avg30={stepsSummary?.avg30 ?? null}
                 slope30={stepsSummary?.slope30 ?? null}
+                trend7Delta={summaryToTrend7Delta(stepsSummary)}
                 icon={Footprints}
                 directionSentiment="up-good"
               />