fix(vscode): avoid automatic Shopify login prompts

[efegokdemir]

Fixes #1247

Opening a theme workspace currently triggers shopify theme metafields pull. If Shopify CLI is not authenticated, this can open a browser login during every VS Code startup, even when a user only needs local Liquid editor features.

Automatic metafield fetching is now opt-in through themeCheck.fetchMetafieldDefinitions (default: false). Users who want remote metafield completions can enable it; the existing fetch path remains available.

Regression coverage verifies that the fetch is skipped by default and still runs when the setting is enabled.

Validation:

• vitest run packages/theme-language-server-common/src/server/startServer.spec.ts — 9 tests passed
• Prettier check for changed TypeScript, package JSON, and changeset — passed
• git diff --check — passed
• Extension package.json parses as valid JSON
• Type-check was attempted but could not complete in the isolated worktree because referenced workspace declaration outputs and matching local dependency types were unavailable.