refactor(nix): add structured config support to home-manager module

Convert settings from raw text to structured Nix attrs, following
Hyprland's module pattern. Implementation based 1:1 on Hyprland's
design - all credit to the Hyprland project.

- Add nix/lib.nix with toMango conversion function
- Support nested attrs, lists for duplicate keys
- Add extraConfig, topPrefixes, bottomPrefixes options
- Auto-add exec-once for autostart.sh

Adapted for mangowc syntax (underscore separators vs colons).

nix/lib.nix

@@ -0,0 +1,257 @@
+lib:
+let
+  inherit (lib)
+    attrNames
+    filterAttrs
+    foldl
+    generators
+    partition
+    ;
+
+  inherit (lib.strings)
+    concatMapStrings
+    hasPrefix
+    ;
+
+  /**
+    Convert a structured Nix attribute set into Mango's configuration format.
+
+    This function takes a nested attribute set and converts it into Mango-compatible
+    configuration syntax, supporting top, bottom, and regular command sections.
+
+    Commands are flattened using the `flattenAttrs` function, and attributes are formatted as
+    `key = value` pairs. Lists are expanded as duplicate keys to match Mango's expected format.
+
+    Configuration:
+
+    * `topCommandsPrefixes` - A list of prefixes to define **top** commands (default: `[]`).
+    * `bottomCommandsPrefixes` - A list of prefixes to define **bottom** commands (default: `[]`).
+
+    Attention:
+
+    - The function ensures top commands appear **first** and bottom commands **last**.
+    - The generated configuration is a **single string**, suitable for writing to a config file.
+    - Lists are converted into multiple entries, ensuring compatibility with Mango.
+
+    # Inputs
+
+    Structured function argument:
+
+    : topCommandsPrefixes (optional, default: `[]`)
+      : A list of prefixes that define **top** commands. Any key starting with one of these
+        prefixes will be placed at the beginning of the configuration.
+    : bottomCommandsPrefixes (optional, default: `[]`)
+      : A list of prefixes that define **bottom** commands. Any key starting with one of these
+        prefixes will be placed at the end of the configuration.
+
+    Value:
+
+    : The attribute set to be converted to Hyprland configuration format.
+
+    # Type
+
+    ```
+    toMango :: AttrSet -> AttrSet -> String
+    ```
+
+    # Examples
+    :::{.example}
+
+    ## Basic mangowc configuration
+
+    ```nix
+    let
+      config = {
+        blur = 1;
+        blur_params_radius = 5;
+        border_radius = 6;
+        animations = 1;
+        animation_duration_open = 400;
+      };
+    in lib.toMango {} config
+    ```
+
+    **Output:**
+    ```
+    animations = 1
+    animation_duration_open = 400
+    blur = 1
+    blur_params_radius = 5
+    border_radius = 6
+    ```
+
+    ## Using nested attributes
+
+    ```nix
+    let
+      config = {
+        blur = 1;
+        blur_params = {
+          radius = 5;
+          num_passes = 2;
+          noise = 0.02;
+        };
+        animation_curve = {
+          open = "0.46,1.0,0.29,1";
+          close = "0.08,0.92,0,1";
+        };
+      };
+    in lib.toMango {} config
+    ```
+
+    **Output:**
+    ```
+    animation_curve_close = 0.08,0.92,0,1
+    animation_curve_open = 0.46,1.0,0.29,1
+    blur = 1
+    blur_params_noise = 0.02
+    blur_params_num_passes = 2
+    blur_params_radius = 5
+    ```
+
+    ## Using lists for duplicate keys
+
+    ```nix
+    let
+      config = {
+        bind = [
+          "SUPER,r,reload_config"
+          "Alt,space,spawn,rofi -show drun"
+          "Alt,Return,spawn,foot"
+        ];
+        tagrule = [
+          "id:1,layout_name:tile"
+          "id:2,layout_name:scroller"
+        ];
+      };
+    in lib.toMango {} config
+    ```
+
+    **Output:**
+    ```
+    bind = SUPER,r,reload_config
+    bind = Alt,space,spawn,rofi -show drun
+    bind = Alt,Return,spawn,foot
+    tagrule = id:1,layout_name:tile
+    tagrule = id:2,layout_name:scroller
+    ```
+
+    :::
+  */
+  toMango =
+    {
+      topCommandsPrefixes ? [ ],
+      bottomCommandsPrefixes ? [ ],
+    }:
+    attrs:
+    let
+      toMango' =
+        attrs:
+        let
+          # Specially configured `toKeyValue` generator with support for duplicate keys
+          # and a legible key-value separator.
+          mkCommands = generators.toKeyValue {
+            mkKeyValue = generators.mkKeyValueDefault { } " = ";
+            listsAsDuplicateKeys = true;
+            indent = ""; # No indent, since we don't have nesting
+          };
+
+          # Flatten the attrset, combining keys in a "path" like `"a_b_c" = "x"`.
+          # Uses `flattenAttrs` with an underscore separator.
+          commands = flattenAttrs (p: k: "${p}_${k}") attrs;
+
+          # General filtering function to check if a key starts with any prefix in a given list.
+          filterCommands = list: n: foldl (acc: prefix: acc || hasPrefix prefix n) false list;
+
+          # Partition keys into top commands and the rest
+          result = partition (filterCommands topCommandsPrefixes) (attrNames commands);
+          topCommands = filterAttrs (n: _: builtins.elem n result.right) commands;
+          remainingCommands = removeAttrs commands result.right;
+
+          # Partition remaining commands into bottom commands and regular commands
+          result2 = partition (filterCommands bottomCommandsPrefixes) result.wrong;
+          bottomCommands = filterAttrs (n: _: builtins.elem n result2.right) remainingCommands;
+          regularCommands = removeAttrs remainingCommands result2.right;
+        in
+        # Concatenate strings from mapping `mkCommands` over top, regular, and bottom commands.
+        concatMapStrings mkCommands [
+          topCommands
+          regularCommands
+          bottomCommands
+        ];
+    in
+    toMango' attrs;
+
+  /**
+    Flatten a nested attribute set into a flat attribute set, using a custom key separator function.
+
+    This function recursively traverses a nested attribute set and produces a flat attribute set
+    where keys are joined using a user-defined function (`pred`). It allows transforming deeply
+    nested structures into a single-level attribute set while preserving key-value relationships.
+
+    Configuration:
+
+    * `pred` - A function `(string -> string -> string)` defining how keys should be concatenated.
+
+    # Inputs
+
+    Structured function argument:
+
+    : pred (required)
+      : A function that determines how parent and child keys should be combined into a single key.
+        It takes a `prefix` (parent key) and `key` (current key) and returns the joined key.
+
+    Value:
+
+    : The nested attribute set to be flattened.
+
+    # Type
+
+    ```
+    flattenAttrs :: (String -> String -> String) -> AttrSet -> AttrSet
+    ```
+
+    # Examples
+    :::{.example}
+
+    ```nix
+    let
+      nested = {
+        a = "3";
+        b = { c = "4"; d = "5"; };
+      };
+
+      separator = (prefix: key: "${prefix}.${key}");  # Use dot notation
+    in lib.flattenAttrs separator nested
+    ```
+
+    **Output:**
+    ```nix
+    {
+      "a" = "3";
+      "b.c" = "4";
+      "b.d" = "5";
+    }
+    ```
+
+    :::
+  */
+  flattenAttrs =
+    pred: attrs:
+    let
+      flattenAttrs' =
+        prefix: attrs:
+        builtins.foldl' (
+          acc: key:
+          let
+            value = attrs.${key};
+            newKey = if prefix == "" then key else pred prefix key;
+          in
+          acc // (if builtins.isAttrs value then flattenAttrs' newKey value else { "${newKey}" = value; })
+        ) { } (builtins.attrNames attrs);
+    in
+    flattenAttrs' "" attrs;
+in
+{
+  inherit flattenAttrs toMango;
+}