Restructure system-modules and bump flake.lock

- Consolidate enable flags into a `packages` attribute in all host
system-modules.nix files. - Rename package option paths to
`config.packages.<name>.enable` in modules/core/packages/packages.nix. -
Update locked revisions (lastModified, narHash, rev) for firefox‑addons
and stylix in flake.lock.

flake.lock

@@ -143,11 +143,11 @@
       },
       "locked": {
         "dir": "pkgs/firefox-addons",
-        "lastModified": 1764389080,
-        "narHash": "sha256-BEn1Z9Uv20u2DS6wzLKdzx5kAzynM3wMQ9JnGf3VJvI=",
+        "lastModified": 1764475519,
+        "narHash": "sha256-12TAT2CD+L+wdIHszQnPyu8zGSBxcazoVZmP6UQjk6s=",
         "owner": "rycee",
         "repo": "nur-expressions",
-        "rev": "897437c09bf22ce59efb3370f0783d0c662dba31",
+        "rev": "1172c39fb0e847a7f036e480d94f629edcf2cf46",
         "type": "gitlab"
       },
       "original": {
@@ -523,11 +523,11 @@
         "tinted-zed": "tinted-zed"
       },
       "locked": {
-        "lastModified": 1764254063,
-        "narHash": "sha256-V22JzkaTLF/GAL2LgqvOsJhAr8JbJsKaD8hnHjGwXfE=",
+        "lastModified": 1764451865,
+        "narHash": "sha256-d4tfTm3ccp4rx7W1WW1BzSkTgEeMd4cZvhAJ7lP145M=",
         "owner": "danth",
         "repo": "stylix",
-        "rev": "a1451bc40413870f0c7b576b751c1ca92055e323",
+        "rev": "8a096ccec828c68bfb870295d186ad994ea0ae2c",
         "type": "github"
       },
       "original": {