feat: Add custom backup prefix, improve flash safety UI, and fix CI

[shoey63]

Overview

This PR introduces a few quality-of-life UI improvements for user safety and backup management, alongside a fix for the broken GitHub Actions build pipeline.

Changes Included

• Custom Backup Prefixes: Added a Jetpack Compose dialog when initiating a backup. Users can now input an optional custom string (e.g., "Stock", "Inactive - all options"). The input is strictly regex-sanitized to prevent file system errors and is prepended to the standard timestamp/slot suffix.
• Flash Safety UI: Updated the flash confirmation dialog to explicitly state the Source and Destination Partition. This provides a critical safety check to prevent users from accidentally flashing an image to the wrong block.
• CI Workflow Fix: The recent migration to Kotlin DSL (build.gradle.kts) broke the GitHub Actions pipeline, as the sed commands were still looking for the old app/build.gradle file. Updated the .github/workflows YAML files to target the correct .kts extension, restoring automated CI builds.

Testing

• Compiled locally and successfully built via GitHub Actions.
• Tested backup creation with custom strings containing spaces and special characters (regex successfully formats them to safe underscores).
• Verified the generated folder names parse correctly within the internal backup list UI.

(Screenshots of the new UI below)