Limit birth year input to 4 digits max (brisbanesocialchess#375) (brisbanesocialchess#378)

* Fix birth year input limit to 4 digits in new structure

- Updated frontend/register.md with oninput handler
- Keeps last 4 digits when user types more than 4 digits
- Addresses issue brisbanesocialchess#375 with new site generator structure

* Remove semicolon from oninput attribute per BaseMax feedback

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Co-authored-by: John Bampton <jbampton@users.noreply.github.com>

Author: Aashish-Jha-11

frontend/register.md

@@ -18,7 +18,7 @@ title: Register
         </div>
         <div class="form-group">
             <label for="birthYear">Birth Year <span aria-hidden="true">*</span> </label>
-            <input type="number" id="birthYear" name="birthyear" min="1900" max="2025" required placeholder="e.g. 1990" />
+            <input type="number" id="birthYear" name="birthyear" min="1900" max="2025" oninput="if (this.value.length > 4) this.value = this.value.slice(-4)" required placeholder="e.g. 1990" />
         </div>
         <fieldset class="form-group">
             <legend>Gender <span aria-hidden="true">*</span></legend>