refactor(decnet_web/DeckyFleet): move IntervalEditor out

Verbatim move of the per-decky mutation-interval modal (~60 LOC)
into its own file. Saves null when the toggle is off, minutes
otherwise.

- New DeckyFleet/IntervalEditor.tsx
- IntervalEditor.test.tsx covers null-current disabled path,
  numeric-current enabled path, and CANCEL not firing onSave.
- src/test/fixtures/decky.ts now derives DeckyFixture from the
  canonical Decky type (the fixture's loose swarm shape was
  missing host_address/host_status; aligning to Decky catches
  that statically).

decnet_web/src/components/DeckyFleet.tsx

@@ -1004,67 +1004,7 @@ const DeployWizard: React.FC<DeployWizardProps> = ({
   );
 };
 
-// ─── Interval editor modal ───────────────────────────────────────────────
-
-interface IntervalEditorProps {
-  open: boolean;
-  deckyName: string;
-  current: number | null;
-  onClose: () => void;
-  onSave: (minutes: number | null) => void;
-}
-
-const IntervalEditor: React.FC<IntervalEditorProps> = ({ open, deckyName, current, onClose, onSave }) => {
-  const [enabled, setEnabled] = useState<boolean>(current !== null);
-  const [minutes, setMinutes] = useState<number>(current ?? 30);
-
-  return (
-    <Modal
-      open={open}
-      onClose={onClose}
-      title={`MUTATION INTERVAL · ${deckyName}`}
-      icon={RefreshCw}
-      accent="violet"
-      footer={
-        <>
-          <button className="btn ghost" onClick={onClose}>CANCEL</button>
-          <button className="btn violet" onClick={() => onSave(enabled ? minutes : null)}>SAVE</button>
-        </>
-      }
-    >
-      <div className="modal-body">
-        <div style={{ display: 'flex', gap: 10, alignItems: 'center', padding: 14, border: '1px solid var(--border)' }}>
-          <input
-            id="interval-enable"
-            type="checkbox"
-            checked={enabled}
-            onChange={(e) => setEnabled(e.target.checked)}
-            style={{ accentColor: 'var(--matrix)' }}
-          />
-          <label htmlFor="interval-enable" style={{ fontSize: '0.8rem', letterSpacing: 1 }}>
-            ENABLE PERIODIC MUTATION
-          </label>
-        </div>
-        {enabled && (
-          <div className="tweak-group">
-            <label>INTERVAL ({minutes} minutes)</label>
-            <input
-              type="range"
-              min={5}
-              max={240}
-              step={5}
-              value={minutes}
-              onChange={(e) => setMinutes(parseInt(e.target.value, 10))}
-            />
-            <div className="dim" style={{ fontSize: '0.65rem', letterSpacing: 1 }}>
-              Applied on the next mutation cycle.
-            </div>
-          </div>
-        )}
-      </div>
-    </Modal>
-  );
-};
+import { IntervalEditor } from './DeckyFleet/IntervalEditor';
 
 // ─── Fleet page ──────────────────────────────────────────────────────────