test(thumbnails): coverage floors for the thumbnail modules and WebP in page workflows

The two new frontend modules enter the coverage baseline (98% lines);
other floors only rise. The visual fixture's preview is WebP and
Appearance has its Thumbnails surface in the System shell check.

tests/browser/workspace-visual.spec.mjs

@@ -173,7 +173,8 @@ test('visual fixture exposes real stable media and library records', async ({ re
     await expect(await request.get(image.url)).toBeOK();
     const thumbnail = await request.get(image.thumbnail_url);
     await expect(thumbnail).toBeOK();
-    expect(thumbnail.headers()['content-type']).toBe('image/png');
+    // #317: previews are written as WebP by default.
+    expect(thumbnail.headers()['content-type']).toBe('image/webp');
     const inspection = await (await request.get(`/prompt_manager/images/inspect/${encodeURIComponent(image.path)}?root=${image.root_index}`)).json();
     expect(inspection.success).toBe(true);
     expect(inspection.image.path).toBe(image.path);

tests/browser/workspace.spec.mjs

@@ -469,7 +469,7 @@ test('System is native: the concept shell, section navigation, sections, hand-of
         if (hash === 'overview') await expect(layout.locator(':scope > .summary-metrics')).toBeVisible();
         if (hash === 'sources') await expect(layout.locator('#source-list .operation')).toHaveCount(2);
         expect(await layout.evaluate(node => [...node.children].map(child => `${child.localName}.${child.className}`)))
-            .toEqual({ overview: ['div.summary-metrics', 'div.two-col', 'div.save-row'], appearance: ['section.surface', 'div.save-row'],
+            .toEqual({ overview: ['div.summary-metrics', 'div.two-col', 'div.save-row'], appearance: ['section.surface', 'section.surface', 'div.save-row'],
                 sources: ['div.notice', 'section.surface', 'div.save-row'],
                 maintenance: ['div.notice', 'section.surface handoff', 'div.save-row'] }[hash]
                 ?? ['section.surface handoff', 'div.save-row']);

tools/vendor/frontend-coverage-baseline.json

@@ -1,9 +1,9 @@
 {
   "admin.js": {
-    "lines": 31,
+    "lines": 32,
     "branches": 58,
-    "functions": 26,
-    "statements": 31
+    "functions": 27,
+    "statements": 32
   },
   "api-client.js": {
     "lines": 98,
@@ -188,7 +188,7 @@
   "workspace/file-inspector.mjs": {
     "lines": 96,
     "branches": 93,
-    "functions": 92,
+    "functions": 93,
     "statements": 96
   },
   "workspace/heading.mjs": {
@@ -241,7 +241,7 @@
   },
   "workspace/image-tagging.mjs": {
     "lines": 97,
-    "branches": 90,
+    "branches": 92,
     "functions": 88,
     "statements": 97
   },
@@ -338,7 +338,7 @@
   "workspace/library-location.mjs": {
     "lines": 94,
     "branches": 91,
-    "functions": 92,
+    "functions": 93,
     "statements": 94
   },
   "workspace/library-state.mjs": {
@@ -372,14 +372,14 @@
     "statements": 97
   },
   "workspace/local-resolver-stages.mjs": {
-    "lines": 94,
+    "lines": 95,
     "branches": 93,
     "functions": 98,
-    "statements": 94
+    "statements": 95
   },
   "workspace/local-resolver-support.mjs": {
     "lines": 96,
-    "branches": 80,
+    "branches": 81,
     "functions": 98,
     "statements": 96
   },
@@ -426,10 +426,10 @@
     "statements": 98
   },
   "workspace/prompt-blocks.mjs": {
-    "lines": 90,
-    "branches": 92,
+    "lines": 98,
+    "branches": 95,
     "functions": 98,
-    "statements": 90
+    "statements": 98
   },
   "workspace/prompt-inspector.mjs": {
     "lines": 98,
@@ -487,7 +487,7 @@
   },
   "workspace/system-controls.mjs": {
     "lines": 98,
-    "branches": 87,
+    "branches": 89,
     "functions": 89,
     "statements": 98
   },
@@ -509,6 +509,12 @@
     "functions": 98,
     "statements": 98
   },
+  "workspace/system-thumbnails.mjs": {
+    "lines": 98,
+    "branches": 95,
+    "functions": 83,
+    "statements": 98
+  },
   "workspace/system.mjs": {
     "lines": 98,
     "branches": 92,
@@ -522,10 +528,16 @@
     "statements": 98
   },
   "workspace/thumbnail-review.mjs": {
-    "lines": 96,
+    "lines": 98,
     "branches": 93,
     "functions": 98,
-    "statements": 96
+    "statements": 98
+  },
+  "workspace/thumbnail-support.mjs": {
+    "lines": 98,
+    "branches": 98,
+    "functions": 98,
+    "statements": 98
   },
   "workspace/toast.mjs": {
     "lines": 98,