fix(plan): allow empty notes, use local time for past-date gate

Three related fixes from manual smoke testing:
- Zod schema: drop min(1) on note field across all four schemas — note is
  optional in real usage, and the modal initialized it to empty.
- assertNotPastDate: replace toISOString() (UTC) with local-time getters
  to match the rest of the codebase. Fixes off-by-one near day boundaries.
- createEntry / updateEntry: also check the date_to endpoint for past
  dates, so a range can't extend backwards into the past even when its
  start is today or future. Defense in depth.

Modal: drop `required` attribute and asterisk from the note field. Tests:
add 5 new cases covering empty note, past date HTTP rejection, and the
new to-endpoint check in updateEntry.

src/admin/components/PlanCellModal.tsx

@@ -286,13 +286,12 @@ function EditForm(props: Props) {
             ))}
           </select>
         </FormField>
-        <FormField label="Text poznámky" required>
+        <FormField label="Text poznámky (volitelný)">
           <textarea
             value={note}
             onChange={(e) => setNote(e.target.value)}
             maxLength={500}
             rows={3}
-            required
           />
         </FormField>
       </FormModal>