feat: sparser robot nudges, typed unread badge, lobby unread bump

Three owner-requested polish changes:

- robot: replace the lengthening 60-90 min -> 6 h proactive-nudge ramp with a
  flat uniform 9-12 h wait before every nudge; the existing sleep-window gate
  still skips and defers a nudge that would land in the robot's night.
- ui: colour the lobby/in-game unread dot by type -- the regular danger colour
  when a chat message is unread, a softer amber (--warn) when only nudges are.
  Adds a per-viewer unread_messages flag (chat_messages.kind='message') across
  the backend DTO, FlatBuffers wire, gateway transcode and the UI store.
- ui: float games with any unread notification to the top of the lobby's
  your-turn and opponent-turn sections (finished keeps its order), reusing the
  existing unread_chat flag.

Docs (ARCHITECTURE 7, FUNCTIONAL + _ru) updated. No DB migration; the new wire
field is backward-compatible.

backend/internal/robot/strategy.go

@@ -79,16 +79,14 @@ const (
 	// sleep window relative to the opponent's timezone, in hours.
 	sleepDriftHours = 3
 
-	// The robot proactively nudges the idle human on a lengthening, randomized schedule rather
-	// than an hourly stream: the first nudge lands ~60-90 min into the turn, and each subsequent
-	// gap grows toward 1-6 h the longer the wait drags on, so a long idle turn gets only a handful
-	// of increasingly-spaced reminders. The gap is a uniform sample in [nudgeGapFloorMinutes,
-	// ceil] minutes, where ceil ramps from nudgeGapFirstCeilMinutes to nudgeGapCeilMinutes over
-	// nudgeGapRamp of idle.
-	nudgeGapFloorMinutes     = 60.0
-	nudgeGapFirstCeilMinutes = 90.0
-	nudgeGapCeilMinutes      = 360.0
-	nudgeGapRamp             = 12 * time.Hour
+	// The robot proactively nudges the idle human on a sparse, randomized schedule rather than an
+	// hourly stream: every nudge waits a uniform random 9-12 h after its reference point (the turn
+	// start for the first nudge, the previous nudge thereafter), so even a long-neglected turn
+	// collects only a few widely-spaced reminders. The 3 h window width is the random spread; the
+	// gap does not lengthen with idle time. The driver still skips a nudge that would land in the
+	// robot's sleep window, deferring it to the first scan after wake.
+	nudgeGapLoHours = 9.0
+	nudgeGapHiHours = 12.0
 )
 
 // defaultBand is the target resulting score margin after the robot's move: when
@@ -263,19 +261,13 @@ func nudgeReplyDelay(seed int64, moveCount int) time.Duration {
 
 // proactiveNudgeGap is the randomized wait before the next proactive nudge, given how long the
 // human had already been idle at the previous nudge (refIdle; 0 for the first nudge of the turn).
-// It is a uniform sample in [nudgeGapFloorMinutes, ceil] minutes, where ceil ramps from
-// nudgeGapFirstCeilMinutes (a ~60-90 min first gap) up to nudgeGapCeilMinutes (a 1-6 h gap) as
-// refIdle reaches nudgeGapRamp — so the reminders space out the longer the turn is neglected. It
-// is deterministic per (seed, refIdle), so the driver computes the same due time on every scan.
+// It is a uniform sample in [nudgeGapLoHours, nudgeGapHiHours] hours, deterministic per
+// (seed, refIdle) so the driver computes the same due time on every scan. refIdle only salts the
+// draw, so each successive nudge of a still-idle turn waits a fresh 9-12 h rather than lengthening.
 func proactiveNudgeGap(refIdle time.Duration, seed int64) time.Duration {
-	f := float64(refIdle) / float64(nudgeGapRamp)
-	if f > 1 {
-		f = 1
-	}
-	ceil := nudgeGapFirstCeilMinutes + (nudgeGapCeilMinutes-nudgeGapFirstCeilMinutes)*f
 	u := unitFloat(mix(seed, "pnudge", int(refIdle/(30*time.Minute))))
-	mins := nudgeGapFloorMinutes + (ceil-nudgeGapFloorMinutes)*u
-	return time.Duration(mins * float64(time.Minute))
+	hours := nudgeGapLoHours + (nudgeGapHiHours-nudgeGapLoHours)*u
+	return time.Duration(hours * float64(time.Hour))
 }
 
 // clampMinutes converts a minute count to a duration, clamping it to the hard delay