feat: extend FITS viewer to support PNG and additional FITS extensions

[dgegen]

Pull Request

Description
Extends the FITS file viewer to support PNG files and additional FITS filename extensions (.fit, .fts). When a PNG is opened, the viewer switches to an image-only layout — hiding FITS-specific controls (stretch, log scale, HDU selector) — and displays the file via a native <img> element. Switching between files cancels any in-flight requests via AbortController session management to prevent stale renders.

Changes Made

• Add .fit, .fts, .png to allowed extensions in file_explorer.py
• Add shared/fileTypes.js for extension detection shared between the explorer file list and the viewer
• Add viewer/fileTypes.js bridging window.__astraFileTypes with a safe fallback; exports isPngFile and isFitsFile
• Add PNG display mode in viewer/index.js: applyPngLayout() hides FITS controls, shows <img>; applyFitsLayout() restores the canvas/header layout
• Add load generation counter + AbortController session tracking (beginNewSession, createSessionController) to abort stale requests on file switch
• Expose openViewerFile() on window so the explorer modal delegates file-type routing to the viewer module instead of calling loadPreview directly
• Add fits_url parameter to create_router() and inject window.__ASTRA_FITS_FILES_BASE_PATH for correct raw file URL resolution at non-default mount prefixes
• Move encodePathSegments to basePath.js; add rawFitsUrl() for encoding raw file paths
• Refactor renderer.js: add cancelPending() / enableRender() and a render token to drop stale FITS renders when switching to PNG

How to Test

1. Start the file explorer pointed at a directory containing both .fits and .png files
2. Click a FITS file — verify the normal canvas viewer, header table, HDU selector, and stretch controls appear
3. Click a PNG file — verify the image displays, and stretch/log/HDU controls are hidden/disabled
4. Rapidly click between files — verify no stale image from a previous load flashes in
5. Open a .fit or .fts file — verify it loads correctly as FITS

Checklist

• Code follows project style guidelines
• Tests added/updated
• Documentation updated (if needed)
• All checks pass