Fix media-viewer: src reactivity, error state, configurable alt, CORS fallback, duplicate fetches

[Copilot]

<media-viewer> had several deficiencies: no re-fetch when src changed after mount, blank UI on fetch failure, hardcoded alt="Image", silent CORS failures with no fallback, and duplicate HEAD requests on reconnection.

Changes

• src reactivity: Added updated() hook that resets mimeType/_error and re-fetches when src changes.
• Error state: Added @state() _error — renders <div class="error">Unable to load media</div> instead of a permanently blank loading div on fetch failure.
• Configurable alt: Added @property() alt = 'Image' passed through to <img alt=${this.alt}>.
• CORS/missing Content-Type fallback: Extracted fallbackToExtensionBasedType() — infers video/{ext} for mp4/webm/ogg, falls back to image/unknown otherwise. Called in both the no-header and catch paths.
• Duplicate fetch guard: Added _fetchingFor: string | null flag — set on fetch start, cleared in finally — prevents the double HEAD request that occurs when connectedCallback and updated both fire on initial mount with a src attribute set.

// src changes are now handled reactively
override updated(changedProperties: PropertyValues) {
  if (changedProperties.has('src') && this.src && this._fetchingFor !== this.src) {
    this.mimeType = null;
    this._error = false;
    this.determineFileType();
  }
}

// CORS/network failures now fall back to extension-based type detection
private fallbackToExtensionBasedType() {
  const ext = this.src?.split('.').pop()?.toLowerCase();
  if (ext && ['mp4', 'webm', 'ogg'].includes(ext)) {
    this.mimeType = `video/${ext}`;
  } else {
    this.mimeType = 'image/unknown';
  }
}

Added .error CSS class to media-viewer-styles.ts to match .unsupported styling.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Feature][Medium] Fix media-viewer component deficiencies: src change handling, error state, alt text, CORS fallback</issue_title>
<issue_description>## Summary

The <media-viewer> component has several functional deficiencies that impact reliability and accessibility. These are grouped together as they all affect the same component.

Findings

1. No re-fetch on src property change

File: src/media-viewer/media-viewer.ts, lines 19-22

determineFileType() is only called in connectedCallback(). If the src property changes after the component is connected to the DOM, the MIME type is never re-determined and the component renders stale content.

Fix: Add a updated() lifecycle hook that watches for src changes:

override updated(changedProperties: PropertyValues) {
  if (changedProperties.has('src') && this.src) {
    this.mimeType = null;
    this.determineFileType();
  }
}

2. Blank error state — no visual feedback on failure

File: src/media-viewer/media-viewer.ts, lines 58-65

When the HEAD fetch() fails (network error, CORS restriction), mimeType stays null and the component renders <div class="loading"></div>. The .loading CSS class has no visible content — users see a blank space permanently.

Fix: Add an error state property and render meaningful fallback content:

@state() private _error = false;

// In render():
if (this._error) {
  return html`<div class="error">Unable to load media</div>`;
}

3. Hardcoded alt="Image" — not configurable

File: src/media-viewer/media-viewer.ts, line 70

The <img> element has alt="Image" hardcoded. Consumers cannot provide meaningful alt text for accessibility.

Fix: Add an alt property:

@property() alt = 'Image';
// Use in render:
html`<img src=${this.src} alt=${this.alt} ... />`

4. CORS dependency without fallback

File: src/media-viewer/media-viewer.ts, lines 24-36

The fetch(this.src, { method: 'HEAD' }) call fails silently on CORS-restricted URLs. The component should fall back to file extension detection when the HEAD request fails:

catch (error) {
  // Fallback: determine type from file extension
  const ext = this.src?.split('.').pop()?.toLowerCase();
  if (['mp4', 'webm', 'ogg'].includes(ext || '')) {
    this.mimeType = `video/${ext}`;
  } else {
    this.mimeType = 'image/unknown'; // Default to image rendering
  }
}

5. Duplicate HEAD requests on reconnection

File: src/media-viewer/media-viewer.ts, line 20

connectedCallback() always calls determineFileType() even if mimeType is already set. Add a guard:

override connectedCallback() {
  super.connectedCallback();
  if (!this.mimeType) {
    this.determineFileType();
  }
}
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes [Feature][Medium] Fix media-viewer component deficiencies: src change handling, error state, alt text, CORS fallback #87

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