no-em-dash pre-commit hook for user-facing strings, existing em-dashes rewritten

frontend/scripts/no-em-dash.ts walks src/ with the TypeScript parser and flags
an em-dash in JSX text, string literals and template text. A bare "—"
placeholder passes, separators go in no-em-dash.allow.json. The script is
identical to index's copy.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

frontend/src/utils.ts

@@ -9,8 +9,8 @@ function extractErrorMessage(err: ApiError): string {
   const offline = offlineDetail(err)
   if (offline) {
     return offline.lastSeen
-      ? `Not delivered — the instance is offline (last seen ${ago(offline.lastSeen)})`
-      : "Not delivered — the instance is offline"
+      ? `Not delivered. The instance is offline (last seen ${ago(offline.lastSeen)})`
+      : "Not delivered. The instance is offline"
   }
 
   if (err instanceof AxiosError) {