feat(messages): unread badges + OS notification for private messages

Private messages (NIP-17) arrived in the foreground with no signal: no
unread count, no badge, no OS notification. Hook both into the single
InboxService.ingest choke point.

- UnreadService: per-peer "last read" in the keystore (timestamps+pubkey
  only, no message text), live count via a changes stream, active-peer
  suppression so the open chat never badges or notifies.
- UnreadBadge: reusable, RTL-aware (Flutter Badge) — on the home hamburger,
  the drawer "Chat" item, and per-conversation in the messages list.
- Mark-read on chat open (ChatScreen).
- NotificationService over flutter_local_notifications: generic
  "New message from <name>" (no message text, for privacy), payload = pubkey,
  tap opens /chat/:pubkey. No-op on web/Windows; Android POST_NOTIFICATIONS.
- i18n notifications.newMessageFrom (en/es/pt/ast).

Background/push stays out of scope (foreground-only by design).

Tests: UnreadService, NotificationService (mock plugin), InboxService hooks
(new peer notifies/counts; duplicate, own, and open-chat do not), and a
UnreadBadge widget test. dart analyze clean; commons_core green.

apps/app_seeds/lib/i18n/ast.i18n.json

@@ -214,8 +214,18 @@
     "anyMonth": "Cualquier mes",
     "noDate": "Indicar data de coyecha",
     "monthNames": [
-      "xineru", "febreru", "marzu", "abril", "mayu", "xunu",
-      "xunetu", "agostu", "setiembre", "ochobre", "payares", "avientu"
+      "xineru",
+      "febreru",
+      "marzu",
+      "abril",
+      "mayu",
+      "xunu",
+      "xunetu",
+      "agostu",
+      "setiembre",
+      "ochobre",
+      "payares",
+      "avientu"
     ]
   },
   "lotType": {
@@ -330,30 +340,102 @@
     "some": "dalgunes",
     "plenty": "munches",
     "pinch": "una migaya",
-    "handful": { "singular": "puñáu", "plural": "puñaos" },
-    "teaspoon": { "singular": "cuyaradina", "plural": "cuyaradines" },
-    "spoon": { "singular": "cuyar", "plural": "cuyares" },
-    "cup": { "singular": "taza", "plural": "tazes" },
-    "jar": { "singular": "bote", "plural": "botes" },
-    "sack": { "singular": "sacu", "plural": "sacos" },
-    "packet": { "singular": "sobre", "plural": "sobres" },
-    "cob": { "singular": "panoya", "plural": "panoyes" },
-    "pod": { "singular": "vaina", "plural": "vaines" },
-    "ear": { "singular": "espiga", "plural": "espigues" },
-    "head": { "singular": "cabezuela", "plural": "cabezueles" },
-    "fruit": { "singular": "frutu", "plural": "frutos" },
-    "bulb": { "singular": "bulbu", "plural": "bulbos" },
-    "tuber": { "singular": "tubérculu", "plural": "tubérculos" },
-    "seedHead": { "singular": "cabeza de simiente", "plural": "cabeces de simiente" },
-    "bunch": { "singular": "manoyu", "plural": "manoyos" },
-    "plant": { "singular": "planta", "plural": "plantes" },
-    "pot": { "singular": "tiestu", "plural": "tiestos" },
-    "tray": { "singular": "bandexa", "plural": "bandexes" },
-    "seedling": { "singular": "plántula", "plural": "plántules" },
-    "tree": { "singular": "árbol", "plural": "árboles" },
-    "cutting": { "singular": "esqueje", "plural": "esquejes" },
-    "grams": { "singular": "gramu", "plural": "gramos" },
-    "count": { "singular": "grana", "plural": "granes" }
+    "handful": {
+      "singular": "puñáu",
+      "plural": "puñaos"
+    },
+    "teaspoon": {
+      "singular": "cuyaradina",
+      "plural": "cuyaradines"
+    },
+    "spoon": {
+      "singular": "cuyar",
+      "plural": "cuyares"
+    },
+    "cup": {
+      "singular": "taza",
+      "plural": "tazes"
+    },
+    "jar": {
+      "singular": "bote",
+      "plural": "botes"
+    },
+    "sack": {
+      "singular": "sacu",
+      "plural": "sacos"
+    },
+    "packet": {
+      "singular": "sobre",
+      "plural": "sobres"
+    },
+    "cob": {
+      "singular": "panoya",
+      "plural": "panoyes"
+    },
+    "pod": {
+      "singular": "vaina",
+      "plural": "vaines"
+    },
+    "ear": {
+      "singular": "espiga",
+      "plural": "espigues"
+    },
+    "head": {
+      "singular": "cabezuela",
+      "plural": "cabezueles"
+    },
+    "fruit": {
+      "singular": "frutu",
+      "plural": "frutos"
+    },
+    "bulb": {
+      "singular": "bulbu",
+      "plural": "bulbos"
+    },
+    "tuber": {
+      "singular": "tubérculu",
+      "plural": "tubérculos"
+    },
+    "seedHead": {
+      "singular": "cabeza de simiente",
+      "plural": "cabeces de simiente"
+    },
+    "bunch": {
+      "singular": "manoyu",
+      "plural": "manoyos"
+    },
+    "plant": {
+      "singular": "planta",
+      "plural": "plantes"
+    },
+    "pot": {
+      "singular": "tiestu",
+      "plural": "tiestos"
+    },
+    "tray": {
+      "singular": "bandexa",
+      "plural": "bandexes"
+    },
+    "seedling": {
+      "singular": "plántula",
+      "plural": "plántules"
+    },
+    "tree": {
+      "singular": "árbol",
+      "plural": "árboles"
+    },
+    "cutting": {
+      "singular": "esqueje",
+      "plural": "esquejes"
+    },
+    "grams": {
+      "singular": "gramu",
+      "plural": "gramos"
+    },
+    "count": {
+      "singular": "grana",
+      "plural": "granes"
+    }
   },
   "market": {
     "title": "Simiente cerca de ti",
@@ -436,5 +518,8 @@
     "vouch": "Conozo a esta persona",
     "vouched": "Avales a esta persona",
     "circle": "Nel to círculu"
+  },
+  "notifications": {
+    "newMessageFrom": "Mensaxe nuevu de {name}"
   }
 }