ui: grayscale palette, square corners throughout

Removes all indigo/emerald/amber/red accent colors — everything now
uses the slate ramp. Strips rounded corners from containers, cards,
buttons, inputs, badges; the only remaining rounded element is the
circular logo image itself.

Affordances preserved without color:
- Primary buttons (Publish/Log in/Unlock/Copy link flash): bg-slate-100
  on dark background for high contrast
- Secondary buttons (Copy/Open): bg-slate-800
- Destructive (Delete): outlined with border-slate-500 so it reads as
  different from secondary
- Error messages: uppercase "ERROR" prefix + left border accent
- Success flash / ready drop state: uppercase "PUBLISHED"/"READY" labels
  and solid border-slate-200 vs. the dashed/empty state
- Protected lock: amber badge swapped for a slate-100 badge overlay
- Chart: slate-200 line on slate-400/20 fill instead of indigo

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

internal/server/metrics.go

@@ -109,8 +109,8 @@ func renderChart(series []store.DayPoint) string {
 	}
 	fmt.Fprintf(&area, "L%.1f,%d Z", float64(padLeft)+float64(n-1)*step, padTop+innerH)
 
-	fmt.Fprintf(&b, `<path d="%s" fill="rgb(99 102 241 / 0.15)" stroke="none"/>`, area.String())
-	fmt.Fprintf(&b, `<polyline fill="none" stroke="rgb(99 102 241)" stroke-width="2" points="%s"/>`, pts.String())
+	fmt.Fprintf(&b, `<path d="%s" fill="rgb(148 163 184 / 0.2)" stroke="none"/>`, area.String())
+	fmt.Fprintf(&b, `<polyline fill="none" stroke="rgb(226 232 240)" stroke-width="2" points="%s"/>`, pts.String())
 
 	// x-axis tick labels: first, middle, last day
 	labelIdx := []int{0, n / 2, n - 1}