refactor: separate UI from calibration logic in DoubleCalibrationRecord.vue (#112)

[harsshks]

Summary

Resolves #112

Refactors DoubleCalibrationRecord.vue by extracting calibration math and webcam handling into dedicated modules, reducing the component by ~245 lines and improving separation of concerns.

Changes

New Files

• src/services/CalibrationCalculator.js — Pure utility functions (no Vue dependencies):

  • generateCalibrationPattern() — generates point patterns for 1–9 calibration points
  • generateRuntimePattern() — dynamic pattern based on screen size and store config
  • calculateDistance() — Euclidean distance for eye-blink detection

• src/mixins/CameraMixin.js — Reusable Vue mixin for webcam/MediaRecorder:

  • startWebCamCapture() — camera access with fallback constraints
  • detectFace() — face-landmark detection with auto-retry
  • stopRecord() / stopWebCamCapture() — cleanup

Modified Files

• src/views/DoubleCalibrationRecord.vue — imports and delegates to the new service and mixin instead of inline implementations

Why This Matters

• Testability: CalibrationCalculator.js is pure functions — easy to unit test without mocking Vue
• Reusability: CameraMixin.js can be used by other components that need camera access (e.g., CameraConfiguration.vue)
• Maintainability: Component focuses on UI orchestration, not implementation details

Testing

• ✅ Build compiles successfully (npx vue-cli-service build)
• ⬜ Manual testing with webcam (requires camera + backend)

Dependencies

This PR is based on feat/fullscreen-calibration (PR #103). It should be merged after #103.