Add fish shell support and related configs

Introduce a `shell` variable in host variables and enable the Fish
package.
Make Bash configuration conditional on the selected shell and add Fish
integration for programs such as fzf, starship, kitty, zoxide, and eza.
Provide a full Fish module with aliases, functions, and plugin
placeholders.
Update Stylix to configure Fish, Starship, Fzf, Bat, Anki, Btop, Kitty,
and
Vesktop themes. Remove the large Emoji module and simplify a few Dconf
settings. All changes collectively enable and style the Fish shell
across
the system.

hosts/Garand-Desktop/variables.nix

@@ -15,6 +15,10 @@
   gitUsername = "GarandPLG";
   gitEmail = "garandplg@garandplg.com";
 
+  # User shell
+  # Availabe options: "fish", "bash"
+  shell = "fish";
+
   # Hyprland Settings
   # Examples:
   # extraMonitorSettings = "monitor = Virtual-1,1920x1080@60,auto,1";

hosts/Garand-Laptop/variables.nix

@@ -15,6 +15,10 @@
   gitUsername = "GarandPLG";
   gitEmail = "garandplg@garandplg.com";
 
+  # User shell
+  # Availabe options: "fish", "bash"
+  shell = "fish";
+
   # Hyprland Settings
   # Examples:
   # extraMonitorSettings = "monitor = Virtual-1,1920x1080@60,auto,1";

hosts/default/variables.nix

@@ -15,6 +15,10 @@
   gitUsername = "GarandPLG";
   gitEmail = "my@email.com";
 
+  # User shell
+  # Availabe options: "fish", "bash"
+  shell = "fish";
+
   # Hyprland Settings
   # Examples:
   # extraMonitorSettings = "monitor = Virtual-1,1920x1080@60,auto,1";