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>

website/src/pages/extensions.astro

@@ -20,9 +20,13 @@ const initialItems = sortExtensions(extensionsData.items, 'title');
           <div class="listing-toolbar-row">
             <div class="results-count" id="results-count" aria-live="polite">{initialItems.length} extensions</div>
             <details class="listing-controls">
-              <summary class="listing-controls-trigger btn btn-secondary btn-small">Sort</summary>
+              <summary class="listing-controls-trigger btn btn-secondary btn-small">Sort &amp; Filter</summary>
               <div class="listing-controls-panel">
                 <div class="filters-bar" id="filters-bar">
+                  <div class="filter-group">
+                    <label for="filter-keyword">Keyword:</label>
+                    <select id="filter-keyword" multiple aria-label="Filter by keyword"></select>
+                  </div>
                   <div class="filter-group">
                     <label for="sort-select">Sort:</label>
                     <select id="sort-select" aria-label="Sort extensions">
@@ -30,6 +34,7 @@ const initialItems = sortExtensions(extensionsData.items, 'title');
                       <option value="lastUpdated">Recently Updated</option>
                     </select>
                   </div>
+                  <button id="clear-filters" class="btn btn-secondary btn-small">Clear Filters</button>
                 </div>
               </div>
             </details>