feat: implement services layer with TDD (Milestone 2)

Implemented all core services following strict TDD (RED-GREEN-REFACTOR):

**Services Implemented:**
- EstimateService: Generate estimates with intensity-based range selection
- StorageService: SQLite persistence with auto-pruning
- ShakeDetectionService: Accelerometer-based shake detection

**Features:**
- Cryptographically secure RNG for estimate selection
- Easter egg logic (>15s shake → Humorous mode)
- Intensity-based range calculation (0-0.3: 20%, 0.3-0.7: 50%, 0.7+: 100%)
- SQLite with auto-pruning based on MaxHistorySize
- Shake detection with 1.5g threshold
- Duration tracking and intensity normalization (0.0-1.0)
- Event-based notifications (ShakeDataChanged)

**Tests:**
- EstimateService: 25 tests (RED-GREEN-REFACTOR)
- StorageService: 14 tests (RED-GREEN-REFACTOR)
- ShakeDetectionService: 22 tests (RED-GREEN-REFACTOR)
- Integration tests: 10 tests
- Total: 119 tests, all passing

**Quality:**
- Build: 0 warnings, 0 errors across all platforms
- Coverage: 51.28% line (low due to MAUI template), 87.5% branch
- All service/model code has high coverage

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

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

src/HihaArvio/Services/Interfaces/IEstimateService.cs

@@ -0,0 +1,25 @@
+using HihaArvio.Models;
+
+namespace HihaArvio.Services.Interfaces;
+
+/// <summary>
+/// Service for generating time estimates based on shake data.
+/// </summary>
+public interface IEstimateService
+{
+    /// <summary>
+    /// Generates a time estimate based on shake intensity, duration, and selected mode.
+    /// </summary>
+    /// <param name="intensity">Normalized shake intensity (0.0 to 1.0).</param>
+    /// <param name="duration">Duration of the shake gesture.</param>
+    /// <param name="mode">The estimation mode (Work, Generic, or Humorous).</param>
+    /// <returns>An EstimateResult with the generated estimate and metadata.</returns>
+    /// <remarks>
+    /// Per spec: If duration exceeds 15 seconds, mode is automatically changed to Humorous (easter egg).
+    /// Intensity determines the range of possible estimates:
+    /// - Low (0.0-0.3): narrow range (first 20% of pool)
+    /// - Medium (0.3-0.7): medium range (first 50% of pool)
+    /// - High (0.7-1.0): full range (entire pool)
+    /// </remarks>
+    EstimateResult GenerateEstimate(double intensity, TimeSpan duration, EstimateMode mode);
+}