feat(15-01): fix 13 markdown violations + wire md:lint to fast-checks CI step

- Fix MD040 (11 bare fences): add language tags (text/bash) across 7 files
- Fix MD031 (2 violations): add blank lines around fence in GETTING-STARTED.md
- Wire 'Markdown lint' step to fast-checks job (after Format check, before Typecheck)
- Reformat .markdownlint-cli2.jsonc per Prettier (trailing commas in JSONC)
- pnpm md:lint exits 0; pnpm format:check exits 0; gate can fail on bare fence (verified)

apps/pwa/README.md

@@ -42,7 +42,7 @@ The API backend must also be running for most features. See [GETTING-STARTED.md]
 
 ## Source layout
 
-```
+```text
 src/
   api/          # Typed fetch wrappers for @familysync/api (client.ts, listsClient.ts)
   components/   # Shared UI components co-located with their *.test.tsx files

apps/pwa/e2e/README.md

@@ -73,7 +73,7 @@ Set `DB_PASSWORD` (and other non-default values) via the shell or the repo root
 
 The API enforces this via `apps/api/src/auth/devBypass.ts`:
 
-```
+```text
 if (process.env.NODE_ENV === 'production') → bypass is a no-op (always)
 if (process.env.DEV_AUTH_BYPASS !== 'true') → bypass is a no-op
 ```