Migrate all interactive Blazor components to vanilla JS for full SSR

- Replace server interactivity with vanilla JS (forms.js) for Popover, Calendar, TimePicker, NumberInput, and Counter components

- Rewrite all Razor components to static SSR using data-* attributes for JS hooks

- Simplify InputBase.cs (remove EventCallback, EditContext, SetValueAsync)

- Remove AddInteractiveServerComponents/AddInteractiveServerRenderMode from Program.cs

- Update demo pages: remove @rendermode, replace EditForm with native form

- Add InteractivityGapTests.cs with 30 scoped E2E tests

- Update FormsTests.cs selectors for new static SSR structure

- Fix year picker navigation bug and date format mismatch in forms.js

- All 126 tests passing

Enciphered.Blazor.UIComponents.Demo/Components/Pages/Counter.razor

@@ -1,19 +1,9 @@
 @page "/counter"
-@rendermode InteractiveServer
 
 <PageTitle>Counter</PageTitle>
 
 <h1>Counter</h1>
 
-<p role="status">Current count: @currentCount</p>
+<p role="status">Current count: <span id="counter-value">0</span></p>
 
-<button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
-
-@code {
-    private int currentCount = 0;
-
-    private void IncrementCount()
-    {
-        currentCount++;
-    }
-}
+<button class="btn btn-primary" id="counter-btn" onclick="document.getElementById('counter-value').textContent = ++window._count || (window._count=1)">Click me</button>