Add Obsidian support and update Helium to 0.13.4.1

Introduce a new home module for Obsidian with enable option and BoM
vault.
Expose Obsidian configuration in host‑specific home‑modules files.
Add Stylix theming support for Obsidian (colors, fonts, polarity,
vaults).
Upgrade Helium AppImage to version 0.13.4.1 and update its hash.

modules/core/packages/appimages/helium.nix

@@ -4,8 +4,8 @@
   fetchurl,
 }: let
   pname = "helium";
-  version = "0.13.3.1";
-  hash = "sha256-RS+Sn42V+HjCw41N1zayMVIqlgH+i2B2IdVJwBPmw00=";
+  version = "0.13.4.1";
+  hash = "sha256-z23up+T6bj6F+cQslmI92bEksIAw1OQHRIrmQSaaxY8=";
 
   src = fetchurl {
     url = "https://github.com/imputnet/helium-linux/releases/download/${version}/helium-${version}-x86_64.AppImage";