chore: extract shared dashboard chrome strings to i18n keys

Replaces 47 inline cs()/en ternaries with dashboard.* keys: demo
notices, save/delete failures, delete aria-labels, loading, and
zone/blocked-day copy. Demo fixture names, locale tags, and the
zón/locations plural stay inline (Czech plurals need count-aware
forms).

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FRONTEND_ISSUES.md

@@ -25,7 +25,7 @@
 
 ## Still Needs Frontend Polish
 
-- Dashboard inline i18n ternaries remain (~60 as of M30). Systematic extraction to `i18n.t()` keys is an ongoing task.
+- Dashboard inline i18n ternaries: shared chrome strings extracted to `dashboard.*` keys (demo notices, save/delete failures, delete aria-labels, zone/blocked-day copy). ~56 `cs() ? ...` remain project-wide, of which the dashboard remainder is demo fixture names, `cs-CZ`/`en-US` locale tags, and the `zón`/`locations` plural — all correct inline (Czech plurals need count-aware forms, not a single key).
 - Registration cannot be fully customer-tested locally until Neon Auth environment variables are configured.
 
 ## Fixed This Session (platform audit pass)