feat: Lokale Dateien öffnen und als Datei speichern (D72, Stufe 1)

Zwei Einträge im Dokumenten-Menü: „Datei öffnen…" (Datei-Input, neues
Dokument, Dateiname als Name) und „Als Datei speichern" (Blob-Download,
Dateiname aus dem Dokumentnamen via localfile.js). Läuft in jedem Browser;
Stufe 2 (File System Access, Chromium) folgt.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/examples/werkbaum.werkbaum

@@ -86,7 +86,7 @@
       - [^] #ed.fold.preset: One switch folds everything from size M downward (S)  %% see D44
     - [^] #ed.snaps: Earlier states of a document, every ten minutes (S)  %% only when something changed
       - [^] #ed.snaps.manual: Save a state by hand, before a larger change (XS)  %% ten minutes is the wrong beat for that moment
-    - [ ] #ed.files: Open and save .werkbaum files (S)
+    - [x] #ed.files: Open and save .werkbaum files (S)
     + [^] #ed.fresh: Show what is new since your last visit (S)
       - [^] #ed.fresh.news: A star in the header, with the last few days (S)  %% see D58
     + [?] #ed.percolor: A pastel colour per person (S)
@@ -604,8 +604,9 @@
   unchanged and already at the top of the list.
 
 #ed.files
-  Open a .werkbaum file and save one. Today the leading data format is the one
-  thing that cannot leave the browser as a file, while the diagram can.
+  Open a .werkbaum file and save one, from the document menu. The leading data
+  format was the one thing that could not leave the browser as a file, while
+  the diagram always could.
 
 #ed.fresh
   For documents that come from outside, nodes that went to production since