Fix crash when desktop config JSON is deleted: self-heal from board

server.py and desktop_viewer.py read-only mode required the desktop
config JSONs to exist and crashed/showed "Config-Datei fehlt" if one
was deleted, even though the board already holds the config durably
in NVM. Add versapad_combined.fetch_from_board()/load_or_fetch(): a
missing combined JSON is now transparently rebuilt from the board via
serial and cached back to disk, falling back to the legacy per-profile
JSONs (or a clear error) only when the board is unreachable.

server.py

@@ -1,7 +1,10 @@
 """
 VersaPad Viewer -- Browser-Variante.
-Liest bei jedem Request live die 3 Config-JSONs vom Desktop und rendert
-die Steuermatrix (4x5 Grid + 4 Encoder) je Profil als HTML.
+Liest bei jedem Request die kombinierte Config-JSON vom Desktop und
+rendert die Steuermatrix (4x5 Grid + 4 Encoder) je Profil als HTML. Fehlt
+die JSON (z.B. geloescht), wird sie automatisch per Serial vom Board neu
+aufgebaut und als neuer Cache gespeichert -- das Board ist die eigentliche
+Quelle der Wahrheit, siehe versapad_combined.load_or_fetch().
 Kein Build-Schritt, kein externes Framework -- nur stdlib.
 
 Start: python server.py  [--port 8765]
@@ -12,6 +15,7 @@ import webbrowser
 from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
 from urllib.parse import urlparse, parse_qs
 
+import versapad_combined as vcomb
 import versapad_data as vp
 
 PAGE_CSS = """
@@ -87,7 +91,12 @@ def render_encoder(enc):
 
 
 def render_page(profile):
-    cfg = vp.load_profile(profile)
+    combined = vcomb.load_or_fetch()
+    raw = combined["profiles"][profile]
+    cfg = vp.annotate_profile({
+        "buttons": [dict(b) for b in raw["buttons"]],
+        "encoders": [dict(e) for e in raw["encoders"]],
+    })
     tabs = "".join(
         f'<a class="tab {"active" if p == profile else ""}" href="/?profile={p}">{html.escape(vp.PROFILE_NAMES[p])}</a>'
         for p in sorted(vp.PROFILE_NAMES)
@@ -107,7 +116,7 @@ def render_page(profile):
 <div class="grid">{cells}</div>
 <h2>Encoder</h2>
 <div class="encoders">{encoders}</div>
-<footer>Quelle: {html.escape(vp.CONFIG_PATHS[profile])}</footer>
+<footer>Quelle: {html.escape(vcomb.DEFAULT_PATH)}</footer>
 </body></html>"""
 
 
@@ -130,10 +139,10 @@ class Handler(BaseHTTPRequestHandler):
             self.send_header("Content-Length", str(len(body)))
             self.end_headers()
             self.wfile.write(body)
-        except FileNotFoundError as e:
+        except (FileNotFoundError, RuntimeError) as e:
             self.send_response(500)
             self.end_headers()
-            self.wfile.write(f"Config-Datei fehlt: {e}".encode("utf-8"))
+            self.wfile.write(f"Config nicht verfuegbar: {e}".encode("utf-8"))
 
 
 def main():