feat(ui): email dashboard UX refactor — split-view, row polish, dashboard (PHASES 1/3/4)

Batch of 4 parallel units, rebased onto current develop and integrated:
- PHASE 1 split-view: clicking an email opens an in-place, collapsible + resizable
  reading pane (shared EmailDetail extracted from Senders) instead of a new tab;
  list stays interactive, selection preserved, mobile stacks (<768px); list gains
  Skeleton loading + EmptyState. (FolderView, SearchResults, +EmailDetail, +split.css)
- PHASE 3 row polish: EmailRow gains onOpen (Gmail fallback kept), a new accessible
  Checkbox primitive (ui/checkbox.jsx), 44px rows, clearer hierarchy + hover; keeps
  the search why-matched highlight rendering. (EmailRow, ui/checkbox, styles.css)
- PHASE 4 bulk+keys: BulkToolbar hierarchy/responsive/clear-selection; keyboard nav
  adds ArrowUp/Down + u=unread, hardened input guard. (BulkToolbar, useListKeyboardNav)
- PHASE 3/4 dashboard: remove dead HeatmapWidget, replace CategoryHeatmap with a
  clickable Emails-by-Category bar (a11y: rank by text+count, not colour); Skeleton +
  EmptyState on widgets. (widgets, Dashboard, client.js, styles.css)

Each unit was self-code-reviewed and built green; integrated build passes. Supersedes
PRs #40-43 (their branches had a stale 38-commit base).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/InboxIntel.Api/Controllers/AnalyticsController.cs

@@ -24,6 +24,9 @@ public class AnalyticsController : ApiControllerBase
     public async Task<IActionResult> Volume([FromQuery] int days = 90, CancellationToken ct = default)
         => Ok(await _analytics.GetVolumeOverTimeAsync(UserId, Math.Clamp(days, 1, 3660), ct));
 
+    [HttpGet("heatmap")]
+    public async Task<IActionResult> Heatmap(CancellationToken ct) => Ok(await _analytics.GetHeatmapAsync(UserId, ct));
+
     [HttpGet("category-heatmap")]
     public async Task<IActionResult> CategoryHeatmap(CancellationToken ct) => Ok(await _analytics.GetCategoryHeatmapAsync(UserId, ct));
 

src/InboxIntel.Application/Abstractions/IServices.cs

@@ -22,6 +22,7 @@ public interface IAnalyticsService
     Task<InboxHealthDto> GetInboxHealthAsync(Guid userId, CancellationToken ct = default);
     Task<IReadOnlyList<SenderStatDto>> GetTopSendersAsync(Guid userId, int take = 20, CancellationToken ct = default);
     Task<IReadOnlyList<TimeSeriesPointDto>> GetVolumeOverTimeAsync(Guid userId, int days = 90, CancellationToken ct = default);
+    Task<IReadOnlyList<HeatmapCellDto>> GetHeatmapAsync(Guid userId, CancellationToken ct = default);
     Task<IReadOnlyList<CategoryHeatmapCellDto>> GetCategoryHeatmapAsync(Guid userId, CancellationToken ct = default);
     Task<IReadOnlyList<AttachmentBreakdownDto>> GetAttachmentBreakdownAsync(Guid userId, CancellationToken ct = default);
     Task<SidebarCountsDto> GetSidebarCountsAsync(Guid userId, CancellationToken ct = default);

src/InboxIntel.Application/DTOs/AnalyticsDtos.cs

@@ -12,6 +12,7 @@ public record InboxHealthDto(
 
 public record TimeSeriesPointDto(DateOnly Day, int Count);
 
+public record HeatmapCellDto(int DayOfWeek, int Hour, int Count);
 
 /// <summary>Email counts per category per day-of-week, for the category heatmap.</summary>
 public record CategoryHeatmapCellDto(string Category, int DayOfWeek, int Count);
@@ -38,5 +39,6 @@ public record DashboardSummaryDto(
     int UnreadEmails,
     IReadOnlyList<SenderStatDto> TopSenders,
     IReadOnlyList<TimeSeriesPointDto> VolumeOverTime,
+    IReadOnlyList<HeatmapCellDto> Heatmap,
     IReadOnlyList<AttachmentBreakdownDto> AttachmentBreakdown,
     long StorageEstimateBytes);

src/InboxIntel.Infrastructure/Analytics/AnalyticsService.cs

@@ -16,10 +16,11 @@ public class AnalyticsService : IAnalyticsService
         var health = await GetInboxHealthAsync(userId, ct);
         var top = await GetTopSendersAsync(userId, 10, ct);
         var volume = await GetVolumeOverTimeAsync(userId, 90, ct);
+        var heatmap = await GetHeatmapAsync(userId, ct);
         var attachments = await GetAttachmentBreakdownAsync(userId, ct);
 
         return new DashboardSummaryDto(
-            health, health.TotalEmails, health.UnreadEmails, top, volume, attachments,
+            health, health.TotalEmails, health.UnreadEmails, top, volume, heatmap, attachments,
             health.EstimatedStorageBytes);
     }
 
@@ -74,6 +75,19 @@ public class AnalyticsService : IAnalyticsService
             .ToList();
     }
 
+    public async Task<IReadOnlyList<HeatmapCellDto>> GetHeatmapAsync(Guid userId, CancellationToken ct = default)
+    {
+        var raw = await _db.Emails
+            .Where(e => e.UserId == userId)
+            .Select(e => new { e.SentAtUtc })
+            .ToListAsync(ct);
+
+        return raw
+            .GroupBy(x => new { Dow = (int)x.SentAtUtc.DayOfWeek, Hour = x.SentAtUtc.Hour })
+            .Select(g => new HeatmapCellDto(g.Key.Dow, g.Key.Hour, g.Count()))
+            .ToList();
+    }
+
     public async Task<IReadOnlyList<CategoryHeatmapCellDto>> GetCategoryHeatmapAsync(Guid userId, CancellationToken ct = default)
     {
         var raw = await _db.Emails