Canvas manifest implementation for extensions (#2029)

* Add keyword display to extension cards on website

- Add .resource-keywords and .keyword-tag CSS styles for rendering keyword badges
- Update renderExtensionsHtml() to display keywords below extension description
- Keywords now visible on the website extensions page with styled badges
- Regenerate website data to include keyword metadata

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Canvas manifest implementation for all extensions

Add per-extension canvas manifests with:
- Structured canvas metadata (name, description, version, keywords)
- Screenshot definitions (icon and gallery with path/type)
- Relative paths for images within each extension directory

Enhance extension metadata:
- Generate meaningful descriptions from source analysis
- Extract and assign keywords for discoverability
- Store metadata in package.json and extension source files

Update website rendering and data generation:
- Include keywords in extension cards and search index
- Add per-extension canvas.json files for independent evolution
- Support screenshot metadata in manifest structure
- Generate extensions.json with full canonical paths for website

All 9 local canvas extensions now have complete manifests with descriptions, keywords, and screenshot references.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Tweaking some descriptions

* Fix description priority to prefer package.json over in-source metadata

Reverse the priority in canvasDescription so that package.json descriptions
(which contain the enhanced, manually-curated descriptions) take precedence
over older in-source descriptions extracted from createCanvas(...) calls.

This prevents regression when npm run website:data regenerates outputs,
ensuring that committed canvas.json files maintain the current descriptions.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix canvas validation to skip external.json file

The validation script was treating extensions/external.json as if it were
a directory, causing false validation failures. Added check to skip files
(identified by presence of dot in filename) and only validate actual
canvas extension directories.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/validate-canvas-extensions.yml

@@ -42,7 +42,8 @@ jobs:
               if (parts[0] === EXTENSIONS_DIR && parts.length >= 2) {
                 const extName = parts[1];
                 // Skip the external-assets directory — it's not a canvas extension
-                if (extName !== EXTERNAL_ASSETS_DIR) {
+                // Also skip external.json and other files at extensions root level
+                if (extName !== EXTERNAL_ASSETS_DIR && !extName.includes('.')) {
                   changedExtDirs.add(path.join(EXTENSIONS_DIR, extName));
                 }
               }