Add custom protocol for serving local image assets

[ptheofan]

Summary

Implement a custom om-asset: protocol to serve local image assets referenced in markdown documents. This allows markdown files to reference images using relative or absolute filesystem paths, which are resolved and served by the main process instead of failing to load due to CSP restrictions.

Key Changes

• AssetProtocolService (src/main/services/AssetProtocolService.ts): New service that registers and handles the om-asset: protocol. The handler validates file extensions against a whitelist of image types, reads files from disk, and returns appropriate HTTP responses with correct content-type headers.

• Asset Resolver (src/preload/assetResolver.ts): New preload utility that resolves image references from markdown documents to om-asset: URLs. Handles relative paths (resolved against the document directory), absolute paths, and leaves external URLs and data URIs untouched.

• Asset Path Rewriting (src/renderer/utils/assetPaths.ts): New renderer utility that rewrites src and srcset attributes in rendered markdown to use resolved asset URLs. Supports both simple <img> elements and <picture> elements with <source> tags.

• MarkdownViewer Integration (src/renderer/components/MarkdownViewer.ts): Updated to call asset path rewriting after rendering markdown content, using the preload resolver via the Electron API.

• API Exposure (src/preload/preload.ts, src/shared/types/api.ts): Exposed assets.resolve() method through the Electron API bridge to allow the renderer to resolve asset paths.

• Main Process Setup (src/main/index.ts): Register the asset protocol scheme before app ready and handler after app ready.

• CSP Update (index.html): Updated Content-Security-Policy to allow om-asset: scheme for images.

• Constants (src/shared/constants/index.ts): Added ASSET_PROTOCOL_SCHEME constant.

Implementation Details

• The protocol handler restricts served files to image extensions (SVG, PNG, JPEG, GIF, WebP, BMP, ICO, AVIF, APNG) to prevent arbitrary file access.
• Asset URLs are converted between file: and om-asset: schemes to maintain compatibility with Node's path utilities.
• Query strings and fragments are stripped from references before resolution, as filesystem paths don't support them.
• Percent-encoded references are decoded before resolution.
• Comprehensive test coverage for all three layers: protocol handler, asset resolver, and path rewriting.

https://claude.ai/code/session_01T2U6dtaZLWCuztQVRFTj7U