Give Markdown blocks one consistent gap, roomier on desktop, and a zoom-in cursor on images (#7298)

* fix(scripts): recover installs when a dependency patch changed

patch-package cannot apply a patch over a package patched with an earlier
version of it, and without --error-on-fail it exits 0 locally. Checkouts kept
the stale patch silently: the Markdown code block spacing fix never reached
them. The script now fails the install and removes that package so the next
npm install reinstalls it clean with the current patch. A patch-package that
cannot start fails the install without removing anything.

* fix(app): give Markdown blocks one gap and roomier desktop spacing

Every Markdown block is followed by one gap, density.blockGap: 12 on compact
layouts and 16 wider. The library's element styles and the views stacked
outside it (Mermaid segments on native, streaming head and tail) take the same
token, so no block type carries its own margins. Desktop was too tight at 12.

Markdown images show a zoom-in cursor on web, since tapping opens the lightbox.

.github/workflows/ci.yml

@@ -145,6 +145,9 @@ jobs:
       - name: Install dependencies
         run: node scripts/npm-retry.mjs ci
 
+      - name: Verify dependency patch recovery
+        run: node --test scripts/postinstall-patches.test.mjs
+
       - name: Build server stack
         run: npm run build:server