fix(scanner): detect "use server" after leading comments

[7etsuo]

Three "use server" matchers were silently skipping every export in any file with a leading copyright/license banner. Fixed by routing the file-level directive check through a shared helper that handles shebangs and comment prologues.

• Adds hasFileDirective helper in matchers/utils.ts that strips a leading shebang and any line/block comments before testing for the directive
• Routes server-action, server-action-no-auth, and all-server-actions through it for file-level detection
• Adds 12 unit tests; on a banner-prefixed sample, candidate counts go from 0/0/0 to 3/2/3 across the three matchers

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