fix: snippet network table lookup and multisite test suite

[imantsk]

Summary

Fixes shared/core-rooted bugs surfaced while stabilising the test suite, and hardens the PHPUnit + Playwright suites so they pass in both single-site and multisite environments.

Code fixes

• snippet-ops: read an inserted snippet back from its own network table so network snippets are no longer looked up in the wrong table (returned id 0 / network=false).
• REST network scope: resolve an ambiguous network flag fail-closed — any value that is not explicitly falsy is treated as network-scoped, so a forged truthy value can no longer slip past the network-capability check.
• Manage menu: pass the bulk-download payload into snippet resolution instead of reading the request stream inside the resolver, restoring the network-capability guard and making it unit-testable.
• Webpack: resolve output and module paths from the repo root so the build emits to src/dist rather than a stray nested directory.

Test suite

• Enable the subsite snippets menu for multisite unit tests so capability resolution matches a real network install (clears the multisite 403 cascade).
• Correct the shared-network-snippet read test route.
• Read the cloud REST test token from the active connection.
• Derive the Playwright plugin slug from .wp-env.json.
• Stabilise Playwright: retry config, extended quicknav setup timeout, save-and-confirm retries, and per-spec state cleanup to remove cross-test contamination.
• Ignore a local .env file.

Testing

• PHPUnit single-site: 106 passing.
• PHPUnit multisite (WP_MULTISITE=1): 106 passing.
• phpcs (WPCS) and eslint: clean on all changed files.
• npm run build: compiles successfully to src/dist.