frontend: light-mode logo via CSS invert + prescriptions search

- sidebar + auth logo now invert to black in light mode (dark unchanged),
  same trick the docs site uses — no second asset needed
- prescriptions page: search by patient, file number, medication,
  prescriber or status, with no-match / empty states

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

frontend/lib/i18n/locales/en/translation.json

@@ -258,6 +258,9 @@
     "new": "New prescription",
     "recent": "Recent prescriptions",
     "recentDescription": "Most recent first",
+    "searchPlaceholder": "Search patient, medication, prescriber",
+    "noMatches": "No prescriptions match your search.",
+    "emptyList": "No prescriptions yet.",
     "addFailedTitle": "Couldn't add prescription",
     "addFailedBody": "Please try again.",
     "kpi": {