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/test/fixtures/decky.ts

@@ -1,21 +1,6 @@
-export interface DeckyFixture {
-  name: string;
-  ip: string;
-  services: string[];
-  distro: string;
-  hostname: string;
-  archetype: string | null;
-  service_config: Record<string, Record<string, unknown>>;
-  mutate_interval: number | null;
-  last_mutated: number;
-  swarm?: {
-    host_uuid: string;
-    host_name: string;
-    state: string;
-    last_error: string | null;
-    last_seen: string | null;
-  };
-}
+import type { Decky } from '../../components/DeckyFleet/types';
+
+export type DeckyFixture = Decky;
 
 export const makeDecky = (overrides: Partial<DeckyFixture> = {}): DeckyFixture => ({
   name: 'decoy-01',