feat(web): add Board page with Kanban columns and task cards

Server-rendered Kanban board with 4 status columns (Pending, Active,
Paused, Completed), task cards with category colors and elapsed time,
filter bar with category chips and subtask toggle, and inline create
task form. All handlers support htmx partial updates for status
transitions (start, pause, resume, complete, abandon, create).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

TaskTracker.Api/Pages/Partials/_KanbanColumn.cshtml

@@ -0,0 +1,33 @@
+@using TaskTracker.Api.Pages
+@using TaskTracker.Core.Enums
+@model ColumnViewModel
+
+<div class="kanban-column">
+    <div class="kanban-column-header">
+        <span class="kanban-column-dot" style="background: @Model.Color"></span>
+        <span class="kanban-column-title">@Model.Label</span>
+        <span class="kanban-column-count">@Model.Tasks.Count</span>
+    </div>
+    <div class="kanban-column-bar" style="background: @(Model.Color)33"></div>
+    <div class="kanban-column-body"
+         data-status="@Model.Status"
+         id="column-@Model.Status">
+        @if (Model.Tasks.Count > 0)
+        {
+            @foreach (var task in Model.Tasks)
+            {
+                <partial name="Partials/_TaskCard" model="task" />
+            }
+        }
+        else
+        {
+            <div class="kanban-column-empty">
+                <span>No tasks</span>
+            </div>
+        }
+    </div>
+    @if (Model.Status == WorkTaskStatus.Pending)
+    {
+        <partial name="Partials/_CreateTaskForm" />
+    }
+</div>