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/EstimateService.cs

@@ -0,0 +1,86 @@
+using System.Security.Cryptography;
+using HihaArvio.Models;
+using HihaArvio.Services.Interfaces;
+
+namespace HihaArvio.Services;
+
+/// <summary>
+/// Service for generating time estimates based on shake data.
+/// Implements intensity-based range selection and easter egg logic.
+/// </summary>
+public class EstimateService : IEstimateService
+{
+    // Per spec: Work mode estimates (wider ranges)
+    private static readonly string[] WorkEstimates =
+    {
+        // Gentle shake range (first 20%)
+        "2 hours", "4 hours",
+        // Medium range (first 50%)
+        "1 day", "2 days", "3 days", "5 days", "1 week",
+        // Full range (entire pool)
+        "15 minutes", "30 minutes", "1 hour", "2 weeks", "1 month", "3 months", "6 months", "1 year"
+    };
+
+    // Per spec: Generic mode estimates (wider ranges)
+    private static readonly string[] GenericEstimates =
+    {
+        // Gentle shake range (first 20%)
+        "1 minute", "5 minutes", "10 minutes",
+        // Medium range (first 50%)
+        "15 minutes", "30 minutes", "1 hour", "2 hours", "3 hours",
+        // Full range (entire pool)
+        "30 seconds", "6 hours", "12 hours", "1 day", "3 days", "1 week", "2 weeks", "1 month"
+    };
+
+    // Per spec: Humorous mode estimates (easter egg)
+    private static readonly string[] HumorousEstimates =
+    {
+        "5 minutes", "tomorrow", "eventually", "next quarter",
+        "when hell freezes over", "3 lifetimes", "Tuesday", "never", "your retirement"
+    };
+
+    /// <inheritdoc/>
+    public EstimateResult GenerateEstimate(double intensity, TimeSpan duration, EstimateMode mode)
+    {
+        // Per spec: Easter egg - duration > 15 seconds forces Humorous mode
+        if (duration > TimeSpan.FromSeconds(15))
+        {
+            mode = EstimateMode.Humorous;
+        }
+
+        // Select estimate pool based on mode
+        var pool = mode switch
+        {
+            EstimateMode.Work => WorkEstimates,
+            EstimateMode.Generic => GenericEstimates,
+            EstimateMode.Humorous => HumorousEstimates,
+            _ => throw new ArgumentOutOfRangeException(nameof(mode), mode, "Invalid estimate mode")
+        };
+
+        // Calculate range based on intensity (per spec)
+        var rangeSize = intensity switch
+        {
+            < 0.3 => (int)Math.Ceiling(pool.Length * 0.2),  // First 20% (narrow range)
+            < 0.7 => (int)Math.Ceiling(pool.Length * 0.5),  // First 50% (medium range)
+            _ => pool.Length                                 // Entire pool (full range)
+        };
+
+        // Ensure at least one item in range
+        rangeSize = Math.Max(1, rangeSize);
+
+        // Select random estimate from calculated range using cryptographically secure RNG (per spec)
+        var selectedEstimate = SelectRandomFromRange(pool, rangeSize);
+
+        // Create and return EstimateResult with all metadata
+        return EstimateResult.Create(selectedEstimate, mode, intensity, duration);
+    }
+
+    /// <summary>
+    /// Selects a random item from the first N items of the array using cryptographically secure RNG.
+    /// </summary>
+    private static string SelectRandomFromRange(string[] array, int rangeSize)
+    {
+        var index = RandomNumberGenerator.GetInt32(0, rangeSize);
+        return array[index];
+    }
+}