Show mansion tax cover image (#456)

- Add cover.png image
- Set hideHeaderImage to false
- Update image path to cover.png

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[email protected]>

Author: MaxGhenis

app/api/posts.json

@@ -14,8 +14,8 @@
       "daphne-hansell"
     ],
     "filename": "uk-mansion-tax-autumn-budget.md",
-    "image": "uk-mansion-tax/mansion_tax_map_1m.png",
-    "hideHeaderImage": true
+    "image": "uk-mansion-tax/cover.png",
+    "hideHeaderImage": false
   },
   {
     "title": "Analysing alternative income tax and National Insurance reform options for the UK",

app/public/assets/posts/uk-mansion-tax/cover.png

@@ -14,8 +14,8 @@
       "daphne-hansell"
     ],
     "filename": "uk-mansion-tax-autumn-budget.md",
-    "image": "uk-mansion-tax/mansion_tax_map_1m.png",
-    "hideHeaderImage": true
+    "image": "uk-mansion-tax/cover.png",
+    "hideHeaderImage": false
   },
   {
     "title": "Analysing alternative income tax and National Insurance reform options for the UK",

app/public/data/posts.json

@@ -14,8 +14,8 @@
       "daphne-hansell"
     ],
     "filename": "uk-mansion-tax-autumn-budget.md",
-    "image": "uk-mansion-tax/mansion_tax_map_1m.png",
-    "hideHeaderImage": true
+    "image": "uk-mansion-tax/cover.png",
+    "hideHeaderImage": false
   },
   {
     "title": "Analysing alternative income tax and National Insurance reform options for the UK",