chore(chromium): delete broken static Default/Preferences deployment

features/apps/chromium/default.nix was deploying a 204-byte static
Default/Preferences via Home Manager symlink into Chromium's mutable
profile directory. The deployment is structurally broken — Chromium
expects to write that file at runtime, so either the symlink is
silently replaced on first save (losing the static defaults) or the
write fails silently.

The contents are also redundant + incorrect:

  - `extensions.theme.{use_system,use_custom} = false` — already
    superseded by the managed-policy approach (BrowserThemeColor
    overrides any user-installed theme extension regardless).
  - `browser.theme.{color_scheme,user_color} = 2` — hardcoded "dark"
    via Chromium's color_scheme enum, conflicting with the dynamic
    BrowserColorScheme = isLightTheme ? "light" : "dark" set by
    core/system/browser.nix. A user on flexoki-light / summer-day /
    catppuccin-latte / rose-pine / white would have had a static-vs-
    policy mismatch every time.

Removed the entire features/apps/chromium/ directory (default.nix +
config/Default/Preferences) and dropped the import from
features/default.nix. Chromium theming continues to flow through the
system-level managed policy, which is the canonical chromium-on-NixOS
path.

`nix flake check --no-build` clean.

features/apps/chromium/config/Default/Preferences

@@ -1,15 +0,0 @@
-{
-  "extensions": {
-    "theme": {
-      "id": "",
-      "use_system": false,
-      "use_custom": false
-    }
-  },
-  "browser": {
-    "theme": {
-      "color_scheme": 2,
-      "user_color": 2
-    }
-  }
-}

features/apps/chromium/default.nix

@@ -1,8 +0,0 @@
-{ config, pkgs, lib, ... }:
-
-{
-  xdg.configFile."chromium/Default" = {
-    source = ./config/Default;
-    recursive = true;
-  };
-}

features/default.nix

@@ -15,7 +15,6 @@ in
     ../themes/engine/stylix.nix
     ./apps/alacritty/default.nix
     ./apps/btop/default.nix
-    ./apps/chromium/default.nix
     ./apps/elephant/default.nix
     ./apps/ghostty/default.nix
     ./apps/kitty/default.nix