[TreeView] Add domain count

[imnasnainaec]

Add sense count badge to current domain tile

Changes

Backend:

• Added GetDomainSenseCount(projectId, domainId) endpoint to StatisticsController
• Added CountSensesWithDomain method to WordRepository for efficient MongoDB querying
• Added corresponding service and interface methods

Frontend:

• Display badge with sense count in upper corner of current domain tile
• Badge shows even when count = 0
• Tooltip shows "Number of words gathered in this domain" on hover
• Graceful error handling - failed API calls simply don't show the badge

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This change is 

[github-actions]

⚠️ Commit Message Format Issues ⚠️
commit 24af9473ef:
1: T1 Title exceeds max length (79>72): "Add backend statistics endpoints for domain sense count and progress proportion"

[imnasnainaec]

@copilot Remove from this pr all changes related to showing a progress bar. Keep every change needed to show the number badge on the current domain tile (and to test those changes).

After changing anything in Backend/Controllers/, set up a venv per README instructions (https://github.com/sillsdev/TheCombine?tab=readme-ov-file#python), then with the backend running (npm run backend) do python ./scripts/generate_openapi.py.

[Copilot]

@imnasnainaec I've opened a new pull request, #4047, to work on those changes. Once the pull request is ready, I'll request review from you.