Allow medications with only a generic name (no commercial name required) (#311)

* Initial plan

* feat: allow generic name only for medications (frontend changes)

- Add getMedDisplayName() helper for consistent name display
- Update validation to require either commercial or generic name
- Update all display locations to use display name fallback
- Add i18n keys for nameOrGenericRequired in en.json and de.json
- Remove required attribute from commercial name field
- Update FIELD_LIMITS.name.min from 1 to 0

Co-authored-by: DanielVolz <3275994+DanielVolz@users.noreply.github.com>

* feat: allow generic name only for medications (backend changes)

- Update Zod schema to allow empty name with cross-field refinement
- Update reminder scheduler to use name || genericName for display
- Update planner routes to match medications by display name
- Update existing tests to match new validation behavior

Co-authored-by: DanielVolz <3275994+DanielVolz@users.noreply.github.com>

* fix: update placeholder text and fix FIELD_LIMITS test

- Remove "(optional)" from generic name placeholder in en/de
- Update types.test.ts to expect FIELD_LIMITS.name.min = 0

Co-authored-by: DanielVolz <3275994+DanielVolz@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: DanielVolz <3275994+DanielVolz@users.noreply.github.com>

frontend/src/i18n/de.json

@@ -192,7 +192,7 @@
     },
     "placeholders": {
       "commercial": "z.B. Ozempic",
-      "generic": "z.B. Semaglutid (optional)",
+      "generic": "z.B. Semaglutid",
       "takenBy": "Name eingeben und Enter drücken",
       "addPerson": "Weitere Person hinzufügen...",
       "weight": "z.B. 240",
@@ -436,6 +436,7 @@
     },
     "validation": {
       "required": "Dieses Feld ist erforderlich",
+      "nameOrGenericRequired": "Handelsname oder Wirkstoff ist erforderlich",
       "maxLength": "Maximal {{max}} Zeichen ({{current}}/{{max}})",
       "tooLong": "{{current}}/{{max}} Zeichen"
     },