Refactor constants management to use Pydantic BaseModel for configuration

- Replaced module-level path variables in constants.py with a structured Pydantic Config class.
- Updated all relevant modules (train.py, augmentation.py, exports.py, dataset-visualiser.py, manual_run.py) to access paths through the new config structure.
- Fixed bugs related to image processing and model saving.
- Enhanced test infrastructure to accommodate the new configuration approach.

This refactor improves code maintainability and clarity by centralizing configuration management.

.cursor/skills/refactor/phases/03-safety-net.md

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+# Phase 3: Safety Net
+
+**Role**: QA engineer and developer
+**Goal**: Design and implement tests that capture current behavior before refactoring
+**Constraints**: Tests must all pass on the current codebase before proceeding
+
+## 3a. Design Test Specs
+
+Coverage requirements (must meet before refactoring — see `.cursor/rules/cursor-meta.mdc` Quality Thresholds):
+- Minimum overall coverage: 75%
+- Critical path coverage: 90%
+- All public APIs must have blackbox tests
+- All error handling paths must be tested
+
+For each critical area, write test specs to `REFACTOR_DIR/test_specs/[##]_[test_name].md`:
+- Blackbox tests: summary, current behavior, input data, expected result, max expected time
+- Acceptance tests: summary, preconditions, steps with expected results
+- Coverage analysis: current %, target %, uncovered critical paths
+
+## 3b. Implement Tests
+
+1. Set up test environment and infrastructure if not exists
+2. Implement each test from specs
+3. Run tests, verify all pass on current codebase
+4. Document any discovered issues
+
+**Self-verification**:
+- [ ] Coverage requirements met (75% overall, 90% critical paths)
+- [ ] All tests pass on current codebase
+- [ ] All public APIs have blackbox tests
+- [ ] Test data fixtures are configured
+
+**Save action**: Write test specs; implemented tests go into the project's test folder
+
+**GATE (BLOCKING)**: ALL tests must pass before proceeding to Phase 4. If tests fail, fix the tests (not the code) or ask user for guidance. Do NOT proceed to Phase 4 with failing tests.