feat: Implement drag-and-drop with position persistence for dashboard…

[StuartF303]

… overlays

Fixes drag functionality and adds position persistence for tank and control overlays on the dashboard diagram.

Index.razor (Frontend):

• Replace broken HTML5 drag API with mouse event handlers
  • @onmousedown: Captures overlay and mouse position when drag starts
  • @OnMouseMove: Updates overlay position in real-time during drag
  • @onMouseUp: Saves position to backend when drag ends
• Add ElementReference for diagram container to calculate dimensions
• Add JSRuntime to get container dimensions for percentage calculations
• Add LoadOverlayPositions method to fetch saved positions from API
• Add SaveOverlayPosition method to persist positions to backend
• Update CreateOverlaysFromTanks/Controls to use saved positions
• Add savedPositions dictionary to cache positions locally
• Add BoundingBox class for container dimension calculations
• Add OverlayPositionDto class for API communication
• Fix cursor changes (4-way arrow in edit mode)
• Add user-select: none to prevent text selection during drag

SettingsController.cs (Backend):

• Add ConcurrentDictionary to store overlay positions in memory
• Add GET /api/settings/overlay-positions endpoint
  • Returns all saved overlay positions
• Add POST /api/settings/overlay-positions endpoint
  • Saves or updates overlay position
  • Logs position changes for debugging
• Add DELETE /api/settings/overlay-positions/{id} endpoint
  • Removes saved position for overlay
• Add OverlayPosition class
  • Id: Guid of tank or control
  • Type: "Tank" or "Control"
  • X, Y: Position as percentage (0-100)

Drag Behavior:

• Only works in edit mode (toggle lock/edit icon)
• Smooth dragging with real-time position updates
• Position constrained to 0-100% (stays within container)
• Success notification when position saved
• Positions persist across page reloads and app restarts
• Each tank and control remembers its last saved position

Technical Implementation:

• Mouse events preferred over HTML5 drag API for better Blazor WASM support
• Positions stored as percentages for responsive layout
• In-memory storage (can be persisted to database later)
• Delta-based position calculation for smooth movement
• Container dimensions calculated via JavaScript interop

Fixes:

• Icons now drag smoothly when in edit mode
• Cursor correctly shows 4-way arrow on hover in edit mode
• Positions saved automatically on drag end
• Saved positions loaded on page initialization
• No more broken drag events