Add a repo icon option, rasterized from the randogoth.com favicon

The F-Droid client decodes repo icons as bitmaps, so the site's SVG icon
is shipped as a 512x512 PNG and copied into repo/icons/ on every sync.

module.nix

@@ -34,6 +34,11 @@ let
     repo_name: ${cfg.repoName}
     repo_description: ${cfg.repoDescription}
     repo_keyalias: ${cfg.keyAlias}
+    # fdroidserver checks this path for existence relative to the cwd `fdroid
+    # update` runs in (stateDir), then copies it into repo/icons/ under its
+    # own basename — so this is deliberately already that destination path,
+    # which the sync script keeps populated before every run.
+    repo_icon: repo/icons/icon.png
     keystore: ${cfg.keystoreFile}
     keystorepass: {env: FDROID_KEYSTORE_PASS}
     keypass: {env: FDROID_KEY_PASS}
@@ -69,6 +74,9 @@ let
     # every run. There is nothing secret in ours, but a warning that is always
     # printed is a warning nobody reads when it finally matters.
     install -m 0600 ${repoConfig} ${cfg.stateDir}/config.yml
+    # Kept populated every run rather than once, so an icon change here
+    # propagates on the next sync without anyone having to touch the server.
+    install -D -m 0644 ${cfg.repoIcon} ${cfg.stateDir}/repo/icons/icon.png
     exec ${pythonEnv}/bin/python3 ${./fdroid-sync.py} ${syncConfig} "$@"
   '';
 
@@ -211,6 +219,16 @@ in
       description = "Repo description shown in the F-Droid client.";
     };
 
+    repoIcon = lib.mkOption {
+      type = lib.types.path;
+      default = ./assets/icon.png;
+      description = ''
+        Icon shown next to the repo in the F-Droid client. Must be a raster
+        image (PNG) — the client decodes it as a bitmap, so an SVG renders as
+        a broken icon rather than falling back to anything.
+      '';
+    };
+
     keyAlias = lib.mkOption {
       type = lib.types.str;
       default = "nanuqsaurus";