chore(release): bump to 1.2.15

Version bump across Cargo/tauri/npm/snap/flake, metainfo release entry,
and TAG_DETAILS for 1.2.15. flake.nix hashes cleared (filled post-tag
via nix build once v1.2.15 exists).

cc

src-tauri/Cargo.lock

@@ -5139,7 +5139,7 @@ dependencies = [
 
 [[package]]
 name = "qbz-nix"
-version = "1.2.14"
+version = "1.2.15"
 dependencies = [
  "aes-gcm",
  "anyhow",
@@ -5245,6 +5245,7 @@ dependencies = [
  "anyhow",
  "base64 0.22.1",
  "chrono",
+ "dirs",
  "futures-util",
  "log",
  "md-5",