fix(desktop): keep project Inbox previews compact (#3193)

## Summary

Project pull request and issue previews in Inbox no longer expand when
their content begins with a Markdown heading. Heading-form titles now
match neighboring Inbox typography while preserving the existing
two-line truncation.

### Related issue

Related: #3117

### Testing

- `pnpm build:e2e`
- `pnpm exec playwright test tests/e2e/project-inbox.spec.ts
--project=smoke --retries=0` — passed
- `pnpm exec biome check tests/e2e/project-inbox.spec.ts
src/shared/styles/globals/markdown.css` — passed
- `pnpm check:px-text` — passed
- Captured `desktop/test-results/inbox-preview/01-project-preview.png`

The repository-wide desktop file-size gate remains blocked by the
pre-existing 1,026-line `AgentCreationPreview.tsx` on `main`; the change
itself introduces no file-size regression.

Signed-off-by: Thomas Petersen <thomasp@squareup.com>

desktop/src/shared/styles/globals/markdown.css

@@ -118,6 +118,10 @@
   display: -webkit-box;
   -webkit-box-orient: vertical;
   -webkit-line-clamp: 2;
+  font-size: inherit;
+  font-weight: inherit;
+  letter-spacing: inherit;
+  line-height: inherit;
   overflow: hidden;
 }