feat(ui): shared toast, confirm, dialog, copy and IME-safe Enter primitives

One implementation each, for every page to use instead of its own copy:

- lib/toast.ts + components/Toaster.tsx: toast.success|error|info from
  anywhere, rendered in polite/assertive live regions; an error stays until
  dismissed, a success fades, a repeat collapses.
- lib/confirm.ts + components/ConfirmHost.tsx: await confirmDialog({...})
  and confirmDiscard() through the themed ConfirmModal, replacing
  window.confirm and the actions that asked nothing.
- hooks/useDialog.ts: the modal focus contract (focus in, Tab trapped,
  Escape, focus restored), extracted from ConfirmModal; dialogs stack so
  only the top one answers keys. ConfirmModal now uses it.
- hooks/useArmed.ts: two-click confirmation that disarms after 3 s.
- hooks/useUnsavedGuard.ts: beforeunload while a draft is dirty.
- lib/clipboard.ts copyText: Clipboard API with an execCommand fallback for
  non-secure origins, outcome toasted.
- lib/keys.ts isSubmitEnter: ignores the Enter that confirms an IME
  candidate (Vietnamese Telex, CJK).

client/src/i18n/locales/en/common.json

@@ -188,5 +188,15 @@
     }
   },
   "examples": "Examples",
-  "help": "Help"
+  "help": "Help",
+  "copied": "Copied",
+  "copyFailed": "Could not copy to the clipboard",
+  "dismiss": "Dismiss",
+  "confirm": "Confirm",
+  "unsaved": {
+    "title": "Discard unsaved changes?",
+    "message": "Your edits have not been saved and will be lost.",
+    "discard": "Discard",
+    "keep": "Keep editing"
+  }
 }

client/src/i18n/locales/vi/common.json

@@ -188,5 +188,15 @@
     }
   },
   "examples": "Ví dụ",
-  "help": "Trợ giúp"
+  "help": "Trợ giúp",
+  "copied": "Đã sao chép",
+  "copyFailed": "Không sao chép được vào bộ nhớ tạm",
+  "dismiss": "Đóng",
+  "confirm": "Xác nhận",
+  "unsaved": {
+    "title": "Bỏ các thay đổi chưa lưu?",
+    "message": "Các chỉnh sửa của bạn chưa được lưu và sẽ bị mất.",
+    "discard": "Bỏ thay đổi",
+    "keep": "Tiếp tục sửa"
+  }
 }