feat: Add Spotless ktlint rules and .editorconfig

This commit introduces Spotless ktlint rules for Kotlin files in `app/build.gradle.kts` and adds a new `.editorconfig` file.

Key changes:
- In `app/build.gradle.kts`:
    - Added Spotless `ktlint("1.6.0")` configuration.
    - Included custom rule sets for Compose: `io.nlopez.compose.rules:ktlint:0.4.16`.
    - Disabled the `compose:modifier-missing-check` rule via `editorConfigOverride`.
    - Added `trimTrailingWhitespace()` and `endWithNewline()` formatting options.
- Added a new `.editorconfig` file with settings for:
    - `indent_size = 4`
    - `max_line_length = 100`
    - `compose_check_modifiers_for_visibility = only_public`
    - `ktlint_function_naming_ignore_when_annotated_with = Composable`
    - `ktlint_compose_modifier-missing-check = disabled`