feat: persist composer drafts in memory across navigation

src/app/components/CalendarEventForm.svelte

@@ -20,6 +20,7 @@
   import EditorContent from "@app/editor/EditorContent.svelte"
   import {PROTECTED} from "@app/core/state"
   import {makeEditor} from "@app/editor"
+  import {getDraft, setDraft, clearDraft} from "@app/util/drafts"
   import {pushToast} from "@app/util/toast"
   import {canEnforceNip70} from "@app/core/commands"
 
@@ -37,7 +38,13 @@
     }
   }
 
-  const {url, h, header, initialValues}: Props = $props()
+  let {url, h, header, initialValues}: Props = $props()
+
+  const draftKey = `calendar:${url}:${h ?? ""}`
+  const draft = getDraft(draftKey)
+  if (!initialValues) {
+    initialValues = draft as unknown as typeof initialValues
+  }
 
   const shouldProtect = canEnforceNip70(url)
 
@@ -95,17 +102,34 @@
 
     pushToast({message: "Your event has been saved!"})
     publishThunk({event, relays: [url]})
+    clearDraft(draftKey)
     history.back()
   }
 
-  const content = initialValues?.content || ""
-  const editor = makeEditor({url, submit, uploading, content})
+  const onChange = (json: unknown) => {
+    setDraft(draftKey, {editorContent: json, title, location})
+  }
+  const editor = makeEditor({
+    url,
+    submit,
+    uploading,
+    onChange,
+    content: draft.editorContent ?? initialValues?.content ?? "",
+  })
 
-  let title = $state(initialValues?.title || "")
-  let location = $state(initialValues?.location || "")
+  let title = $state(initialValues?.title ?? "")
+  let location = $state(initialValues?.location ?? "")
   let start: number | undefined = $state(initialValues?.start)
   let end: number | undefined = $state(initialValues?.end)
-  let endDirty = Boolean(initialValues?.end)
+  let endDirty = $state(Boolean(initialValues?.end))
+
+  $effect(() => {
+    setDraft(draftKey, {
+      ...getDraft(draftKey),
+      title,
+      location,
+    })
+  })
 
   $effect(() => {
     if (!endDirty && start) {