easyeffects: add global settings and paired presets

[Tenshock]

Description

Add services.easyeffects.settings for declaratively managing global EasyEffects 8 settings.

Settings are grouped by KConfig section and written to EasyEffects' mutable database during Home Manager activation using kwriteconfig6. Boolean, integer, float, string, and path values are supported. Setting a value to null deletes the corresponding key.

Using activation instead of xdg.configFile keeps the database writable by EasyEffects. An assertion rejects non-empty settings when using EasyEffects versions older than 8.0.0.

Also allow one services.easyeffects.extraPresets.<name> entry to contain both input and output blocks. Each block is generated as a separate preset file in its corresponding directory:

$XDG_DATA_HOME/easyeffects/input/<name>.json
$XDG_DATA_HOME/easyeffects/output/<name>.json

Existing one-sided input or output presets remain compatible.

Finally, extend services.easyeffects.preset with a structured form for selecting different startup presets for the input and output pipelines:

services.easyeffects.preset = {
  input = "voice";
  output = "music";
};

The existing string form remains supported. The structured form requires EasyEffects 8.0.0 or later and must select at least one pipeline; an empty string leaves that pipeline unchanged. After the service starts, Home Manager sends the pipeline-specific load commands to EasyEffects' local service socket.

Tests cover:

• String and boolean global settings.
• Key deletion using null.
• Generated activation commands.
• Unsupported EasyEffects version assertions.
• One-sided and paired input/output preset generation with exact JSON output.
• Backwards-compatible string startup presets.
• Pipeline-specific startup preset commands.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -A dev --run treefmt.

• Code tested through nix build .#test-all
  and targeted nix run .#tests -- easyeffects.

• Test cases updated/added.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}