fix(ux): land quick-win UX audit fixes across settings, exports, toasts, nav

Batch of safe, isolated fixes from the 10-surface UX audit:

- settings: Auto-shield / Save-z-transactions toggles now persist on change
  (they discarded CB()'s return and relied on a Save button that only renders
  on the About tab, so the choice was silently lost on relaunch).
- exports: show the resolved Platform::getConfigDir() instead of a hardcoded
  ~/.config path (wrong on Windows/macOS) in both the all-keys and CSV
  dialogs; all-keys also reveals the file path on a partial export.
- notifications: error/warning toasts last 9s/7s (were 4s/3.5s) so failures
  are readable.
- qr popup / tx-details: copy-to-clipboard now confirms with a toast.
- about: Escape-to-close + readable heading contrast via OnSurfaceMedium().
- sidebar: suppress single-key nav (J/K/arrows/[) while an overlay dialog or
  ImGui popup is open, so it no longer switches the tab behind a modal.
- i18n: strip the stale hardcoded path out of file_save_location in i18n.cpp
  and all eight res/lang/*.json locales (additive, one key each).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

res/lang/de.json

@@ -876,7 +876,7 @@
     "fee_normal": "Normal",
     "fetch_prices": "Preise abrufen",
     "file": "Datei",
-    "file_save_location": "Datei wird gespeichert in: ~/.config/ObsidianDragon/",
+    "file_save_location": "Datei wird gespeichert in:",
     "filter": "Filtern...",
     "font_scale": "Schriftgröße",
     "from": "Von",

res/lang/es.json

@@ -876,7 +876,7 @@
     "fee_normal": "Normal",
     "fetch_prices": "Obtener precios",
     "file": "Archivo",
-    "file_save_location": "El archivo se guardará en: ~/.config/ObsidianDragon/",
+    "file_save_location": "El archivo se guardará en:",
     "filter": "Filtrar...",
     "font_scale": "Escala de fuente",
     "from": "Desde",

res/lang/fr.json

@@ -876,7 +876,7 @@
     "fee_normal": "Normal",
     "fetch_prices": "Récupérer les prix",
     "file": "Fichier",
-    "file_save_location": "Le fichier sera enregistré dans : ~/.config/ObsidianDragon/",
+    "file_save_location": "Le fichier sera enregistré dans :",
     "filter": "Filtrer...",
     "font_scale": "Taille de police",
     "from": "De",

res/lang/ja.json

@@ -876,7 +876,7 @@
     "fee_normal": "通常",
     "fetch_prices": "価格を取得",
     "file": "ファイル",
-    "file_save_location": "ファイルの保存先：~/.config/ObsidianDragon/",
+    "file_save_location": "ファイルの保存先：",
     "filter": "フィルター...",
     "font_scale": "フォントサイズ",
     "from": "送信元",

res/lang/ko.json

@@ -876,7 +876,7 @@
     "fee_normal": "보통",
     "fetch_prices": "가격 조회",
     "file": "파일",
-    "file_save_location": "파일 저장 위치: ~/.config/ObsidianDragon/",
+    "file_save_location": "파일 저장 위치:",
     "filter": "필터...",
     "font_scale": "글꼴 크기",
     "from": "보낸 곳",

res/lang/pt.json

@@ -876,7 +876,7 @@
     "fee_normal": "Normal",
     "fetch_prices": "Buscar preços",
     "file": "Arquivo",
-    "file_save_location": "O arquivo será salvo em: ~/.config/ObsidianDragon/",
+    "file_save_location": "O arquivo será salvo em:",
     "filter": "Filtrar...",
     "font_scale": "Escala da Fonte",
     "from": "De",

res/lang/ru.json

@@ -876,7 +876,7 @@
     "fee_normal": "Обычная",
     "fetch_prices": "Получить цены",
     "file": "Файл",
-    "file_save_location": "Файл будет сохранён в: ~/.config/ObsidianDragon/",
+    "file_save_location": "Файл будет сохранён в:",
     "filter": "Фильтр...",
     "font_scale": "Масштаб шрифта",
     "from": "От",

res/lang/zh.json

@@ -876,7 +876,7 @@
     "fee_normal": "普通",
     "fetch_prices": "获取价格",
     "file": "文件",
-    "file_save_location": "文件将保存至：~/.config/ObsidianDragon/",
+    "file_save_location": "文件将保存至：",
     "filter": "筛选...",
     "font_scale": "字体大小",
     "from": "从",

res/themes/ui.toml

@@ -1229,8 +1229,8 @@ glass-border-alpha = { size = 35.0 }
 progress-bar-height = { size = 2.0 }
 duration-info = { size = 1.0 }
 duration-success = { size = 2.5 }
-duration-warning = { size = 3.5 }
-duration-error = { size = 4.0 }
+duration-warning = { size = 7.0 }
+duration-error = { size = 9.0 }
 window = { width = 350, height = 60, border-radius = 8, border-width = 1 }
 viewport-padding = { size = 20.0 }
 stack-gap = { size = 10.0 }