Address #123 review: viewers can read plop notes; tighten the API

- The "add note" affordance no longer hides from viewers (it claimed
  parity with the bed inspector but gated on !readOnly). A viewer now sees
  "📓 Notes about this plant" and can open the plop's journal to read it;
  the composer stays edit-gated, so they can't write. Real parity now.
- onScopePlantingChange is required, matching onScopeChange (its bed twin),
  so a caller can't pass a plop scope with no way to clear it. Dropped the
  now-dead guard on the "Show all" button.
- Pulled the plop-over-bed scope-label priority into one `scopeLabel`,
  shared by the filter's sibling logic and the composer, so they can't
  drift; trimmed the invariant comment that was restated a third time.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

web/src/editor/PlopInspector.tsx

@@ -35,7 +35,9 @@ export function PlopInspector({
   onChangePlant: () => void
   onClose: () => void
   /** Scope the journal to this plop and open it — the plop parallel of the bed
-   *  inspector's "add note" (#85). Absent for a viewer, who can't write notes. */
+   *  inspector's "add note" (#85). Offered to viewers too (to READ the plop's
+   *  notes, like the bed inspector does); the journal's composer is separately
+   *  gated on edit rights, so a viewer just sees the entries. */
   onAddNote?: () => void
   readOnly?: boolean
 }) {
@@ -176,9 +178,9 @@ export function PlopInspector({
         />
       </fieldset>
 
-      {!readOnly && onAddNote && (
+      {onAddNote && (
         <Button variant="ghost" className="justify-start px-2 py-1 text-sm" onClick={onAddNote}>
-          📓 Add a note about this plant
+          📓 {readOnly ? 'Notes about this plant' : 'Add a note about this plant'}
         </Button>
       )}