fix(l10n): align the stores dialog with the setup trust hint (Sie-form, plain language)

The regulated setup path sends the operator to the stores dialog to
pin a publisher key — but the field was labelled 'Angepinnter
Public-Key — PEM (optional)' with du-form developer prose (PEM,
vendor key) at exactly the trust-critical moment. Field now reads
'Signatur-Schlüssel anheften (PEM, optional)' with a plain-language
Sie-form explanation of what pinning does; the dialog intro and the
onboarding checklist follow (MCP jargon explained in the sentence,
developer-only follow-up cards labelled as such).

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/l10n/app_en.arb

@@ -378,8 +378,8 @@
   "storesManagerRemoveFailed": "Could not remove store",
   "storesManagerSuggested": "Suggested stores",
   "storesManagerAddShort": "Add",
-  "storesManagerPinnedKeyLabel": "Pinned public key — PEM (optional)",
-  "storesManagerPinnedKeyHint": "Signed modules from this store verify against this key instead of the built-in vendor key. Paste a PEM public key.",
+  "storesManagerPinnedKeyLabel": "Pin a signing key (PEM, optional)",
+  "storesManagerPinnedKeyHint": "Only modules signed with this key are accepted from this source. Paste the publisher's public key (PEM) — used instead of the built-in vendor key.",
   "storesManagerNotAvailable": "Not available yet",
   "storesManagerNotAvailableHint": "The provider hasn't published this store index yet. It will become installable once they do.",
   "storesSuggestedReclaimDesc": "Recl∆Im — modules for law & duty reform-mapping",