feat(flutter): add action buttons to desktop download notifications

Terminal task notifications now carry actionable buttons and respond to
clicks instead of being inert:

- Done notifications offer Open File and Open Folder for single-file
  tasks, and Open Folder only for folder tasks (e.g. multi-file
  torrents), following the same rule as the task details page. Error
  notifications offer Open Folder only, since the output is rarely
  usable.
- Clicking the notification body brings the app window back to the
  front (show + focus), covering the close-to-tray case.
- The click target is snapshotted into the notification (Windows: action
  arguments; macOS/Linux: notification payload) so buttons keep working
  after the task record is deleted, and a missing file is logged instead
  of surfacing an OS error dialog.
- macOS resolves buttons through three pre-registered notification
  categories (done single-file / done folder / error); Windows uses the
  actions API of the pinned notification plugin fork; Linux uses
  freedesktop notification actions.
- New openFolder label localized in all 22 ARB catalogs.

Cold-start activation is intentionally out of scope: when the process is
not running, OS activation opens the main window without dispatching the
button action.

ui/flutter/lib/l10n/app_ar.arb

@@ -239,6 +239,7 @@
   "back": "رجوع",
   "open": "فتح",
   "openFile": "فتح الملف",
+  "openFolder": "فتح المجلد",
   "share": "مشاركة",
   "copy": "نسخ",
   "copied": "مُزَوَّل",

ui/flutter/lib/l10n/app_az.arb

@@ -253,6 +253,7 @@
   "back": "Geri",
   "open": "Aç",
   "openFile": "Faylı Aç",
+  "openFolder": "Qovluğu Aç",
   "share": "Paylaş",
   "copy": "Kopyala",
   "copied": "Kopyalandı",

ui/flutter/lib/l10n/app_ca.arb

@@ -239,6 +239,7 @@
   "back": "Enrere",
   "open": "Obre",
   "openFile": "Obre el fitxer",
+  "openFolder": "Obre la carpeta",
   "share": "Comparteix",
   "copy": "Copia",
   "copied": "Copiat",

ui/flutter/lib/l10n/app_de.arb

@@ -239,6 +239,7 @@
   "back": "Zurück",
   "open": "Offen",
   "openFile": "Datei öffnen",
+  "openFolder": "Ordner öffnen",
   "share": "Teilen",
   "copy": "Kopie",
   "copied": "Kopiert",

ui/flutter/lib/l10n/app_en.arb

@@ -261,6 +261,7 @@
   "back": "Back",
   "open": "Open",
   "openFile": "Open File",
+  "openFolder": "Open Folder",
   "share": "Share",
   "copy": "Copy",
   "copied": "Copied",

ui/flutter/lib/l10n/app_es.arb

@@ -239,6 +239,7 @@
   "back": "Volver",
   "open": "Abrir",
   "openFile": "Abrir archivo",
+  "openFolder": "Abrir carpeta",
   "share": "Compartir",
   "copy": "Copiar",
   "copied": "Copiado",

ui/flutter/lib/l10n/app_fa.arb

@@ -239,6 +239,7 @@
   "back": "بازگشت",
   "open": "باز کردن",
   "openFile": "باز کردن فایل",
+  "openFolder": "باز کردن پوشه",
   "share": "اشتراک‌گذاری",
   "copy": "کپی",
   "copied": "اخراج",

ui/flutter/lib/l10n/app_fr.arb

@@ -239,6 +239,7 @@
   "back": "Retour",
   "open": "Ouvrir",
   "openFile": "Ouvrir le fichier",
+  "openFolder": "Ouvrir le dossier",
   "share": "Partager",
   "copy": "Copier",
   "copied": "Copied",

ui/flutter/lib/l10n/app_hu.arb

@@ -239,6 +239,7 @@
   "back": "Vissza",
   "open": "Nyitva",
   "openFile": "Fájl megnyitása",
+  "openFolder": "Mappa megnyitása",
   "share": "Megosztás",
   "copy": "másolat",
   "copied": "Másolva",

ui/flutter/lib/l10n/app_id.arb

@@ -239,6 +239,7 @@
   "back": "Kembali",
   "open": "Buka",
   "openFile": "Buka Berkas",
+  "openFolder": "Buka Folder",
   "share": "Bagikan",
   "copy": "Salin",
   "copied": "Disalin",

ui/flutter/lib/l10n/app_it.arb

@@ -239,6 +239,7 @@
   "back": "Indietro",
   "open": "Apri",
   "openFile": "Apri file",
+  "openFolder": "Apri cartella",
   "share": "Condividi",
   "copy": "Copia",
   "copied": "- Si'",

ui/flutter/lib/l10n/app_ja.arb

@@ -239,6 +239,7 @@
   "back": "戻る",
   "open": "営業部",
   "openFile": "ファイルを開く",
+  "openFolder": "フォルダを開く",
   "share": "共有",
   "copy": "コピー",
   "copied": "コピー",

ui/flutter/lib/l10n/app_ko.arb

@@ -239,6 +239,7 @@
   "back": "뒤로",
   "open": "열기",
   "openFile": "파일 열기",
+  "openFolder": "폴더 열기",
   "share": "공유",
   "copy": "복사",
   "copied": "뚱 베어",

ui/flutter/lib/l10n/app_pl.arb

@@ -239,6 +239,7 @@
   "back": "Wstecz",
   "open": "Otwórz",
   "openFile": "Otwórz plik",
+  "openFolder": "Otwórz folder",
   "share": "Udostępnij",
   "copy": "Kopiuj",
   "copied": "Kopiowane",

ui/flutter/lib/l10n/app_pt.arb

@@ -239,6 +239,7 @@
   "back": "Voltar",
   "open": "Abrir",
   "openFile": "Abrir ficheiro",
+  "openFolder": "Abrir pasta",
   "share": "Partilhar",
   "copy": "Copiar",
   "copied": "Copiado",

ui/flutter/lib/l10n/app_ru.arb

@@ -239,6 +239,7 @@
   "back": "Назад",
   "open": "Открыть",
   "openFile": "Открыть файл",
+  "openFolder": "Открыть папку",
   "share": "Поделиться",
   "copy": "Копия",
   "copied": "Копирован",

ui/flutter/lib/l10n/app_ta.arb

@@ -239,6 +239,7 @@
   "back": "பின்செல்",
   "open": "திற",
   "openFile": "கோப்பைத் திற",
+  "openFolder": "கோப்புறையைத் திற",
   "share": "பகிர்",
   "copy": "நகலெடு",
   "copied": "நகலெடுக்கப்பட்டது",

ui/flutter/lib/l10n/app_tr.arb

@@ -239,6 +239,7 @@
   "back": "Geri",
   "open": "Aç",
   "openFile": "Dosyayı Aç",
+  "openFolder": "Klasörü Aç",
   "share": "Paylaş",
   "copy": "Kopyala",
   "copied": "Kopyalandı",

ui/flutter/lib/l10n/app_uk.arb

@@ -239,6 +239,7 @@
   "back": "Назад",
   "open": "Відкрито",
   "openFile": "Відкрити файл",
+  "openFolder": "Відкрити папку",
   "share": "Поділитися",
   "copy": "Список",
   "copied": "Панчохи",

ui/flutter/lib/l10n/app_vi.arb

@@ -239,6 +239,7 @@
   "back": "Quay lại",
   "open": "Mở",
   "openFile": "Mở tệp",
+  "openFolder": "Mở thư mục",
   "share": "Chia sẻ",
   "copy": "Chép",
   "copied": "Co dãn",

ui/flutter/lib/l10n/app_zh.arb

@@ -239,6 +239,7 @@
   "back": "返回",
   "open": "打开",
   "openFile": "打开文件",
+  "openFolder": "打开文件夹",
   "share": "分享",
   "copy": "复制",
   "copied": "已复制",

ui/flutter/lib/l10n/app_zh_TW.arb

@@ -239,6 +239,7 @@
   "back": "返回",
   "open": "開啟",
   "openFile": "開啟檔案",
+  "openFolder": "開啟資料夾",
   "share": "分享",
   "copy": "複製",
   "copied": "已複製",