Update Flutter wallet SDK install snippet to current release, drop stale general SDK pin - #2810

The Flutter Wallet SDK tutorial pinned stellar_wallet_flutter_sdk: ^1.0.6 alongside stellar_flutter_sdk: ^2.1.3 — a ~year-stale pair where current wallet SDK releases require stellar_flutter_sdk: ^3.x, forcing pub to backtrack to old wallet versions (finding sd-032).

Changes — docs/build/apps/wallet/component/dart/install.mdx

• Pin only the wallet SDK at the current release (verified live against pub.dev: wallet SDK 1.1.4, which depends on stellar_flutter_sdk: ^3.6.0), letting the general SDK resolve transitively — per the maintainer's stated direction. Pinning a single package removes the possibility of documenting an incompatible pair in the future.
• Add a sentence noting the base Flutter Stellar SDK comes in transitively, since later tutorial examples (stellar.mdx) import package:stellar_flutter_sdk directly.
• Fix the code fence language from dart to yaml (it's a pubspec.yaml snippet).

# pubspec.yaml
stellar_wallet_flutter_sdk: ^1.1.4

Only the English tutorial needed the fix — the Spanish translation referenced in the issue was removed with i18n in #2410.

Note for maintainers: per the issue's resolution handoff, notify Raven (sd-032) via the linked upstream-improvement-ready template once deployed.

• Fixes Avoid stale incompatible Flutter wallet SDK version pins #2606