test: add vitest and display module tests

- Install vitest for testing
- Export formatEstimate, isTaskOverdue, formatTags for testing
- Add tests for display helper functions (15 tests)
- Update CI and release workflows to run tests

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: stephendolan

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ on:
     branches: [main]
 
 jobs:
-  check:
+  test:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v5
@@ -20,4 +20,5 @@ jobs:
       - run: npm ci
       - run: npm run typecheck
       - run: npm run lint
+      - run: npm test
       - run: npm run build

.github/workflows/release.yml

@@ -6,7 +6,7 @@ on:
       - 'v*'
 
 jobs:
-  check:
+  test:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v5
@@ -19,9 +19,10 @@ jobs:
       - run: npm ci
       - run: npm run typecheck
       - run: npm run lint
+      - run: npm test
 
   publish:
-    needs: check
+    needs: test
     runs-on: ubuntu-latest
     permissions:
       contents: write