Add batch folder conversion UI and background processing

[bumbitzu]

Motivation

• Provide a folder-based (batch) conversion workflow so users can convert many images by selecting an input and output folder while keeping existing single-file conversion unchanged.
• Keep the UI responsive during batch operations and surface progress/error information to the user.
• Reuse the existing conversion engine and supported-extension list (from Filter.flt) without adding external dependencies.

Description

• Added new WinForms controls to the main window for folder-based conversion: btnSelectInputFolder, txtInputFolder, btnSelectOutputFolder, txtOutputFolder, btnConvertFolder, progressBarBatch, lblBatchStatus, and txtBatchLog (layout updated in MainWindow.Designer.cs).
• Implemented folder selection, validation, and batch pipeline in MainWindow.cs, including: TrySelectFolder, ValidateInputFolder, ValidateOutputFolder, GetInputImages, GetUniqueOutputPath, ConvertSingleImage, and ConvertBatch.
• Discovered supported input extensions by parsing Filter.flt in InitializeSupportedExtensions so the batch file selection matches single-file support; output extension is taken from the currently selected output format radio button.
• Batch conversion runs on a background task (Task.Run) and reports progress using IProgress<T> so UI updates are marshaled to the UI thread; UI controls are disabled during conversion and re-enabled afterwards.
• Error handling: per-file exceptions are collected and shown in txtBatchLog and in a summary MessageBox after the run; output naming avoids overwrites by appending _1, _2, ... when necessary.
• Files modified: MagiConvert/MainWindow.cs (logic and helpers added) and MagiConvert/MainWindow.Designer.cs (UI controls + layout updates).

Testing

• No automated tests were run against these changes.

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Codex Task