feat(conversion): add local media and document quick conversion

[JoeJoeflyn]

Description

Adds native quick media and document conversion directly within Strata, allowing users to convert and compress images, videos, audio, and documents locally without uploading private data to third-party tools.

Key capabilities:

• Image Conversion: WebP, AVIF, PNG, JPEG with quality presets (High, Balanced, Compact) and optional metadata stripping.
• Video & Animation Conversion: MP4 (H.264), WebM (VP9), and animated GIF conversion with quality optimization.
• Audio Extraction & Transcoding: Extract audio from video or transcode audio files to MP3, Opus, AAC.
• Multi-image to PDF: Combine multiple selected images into a single PDF document.
• Context Menu & UI: Context menu integration ("Convert…") with format presets, quality control, output naming, conflict handling, and non-blocking background progress execution.

Visual evidence

N/A (backend operations and standard Strata modal conversion dialog)

How to test

1. Launch Strata and navigate to a folder containing images, videos, or audio files.
2. Select one or more files and right-click to open the context menu.
3. Click Convert… to open the conversion dialog.
4. Select a target format (e.g., WebP, MP4, MP3, or PDF for multiple images), choose a quality preset, and click Convert.

Expected result:
The files convert asynchronously in the background. Once complete, the converted file(s) are highlighted in the directory list.

Related issue

Closes #319