feat(autofill): "Clear the field before typing" setting (default ON)

The Ctrl+A + Del sent before each field misbehaves on targets where Ctrl+A
isn't select-all (terminals, some remote desktops). New synced setting
(autofillClearField, Settings > Autofill) gates it: JS appends clear=0 to
cmd://autofill/execute when off; ExecuteAutofill wraps the three
SendSelectAllAndDelete calls behind AClearFirst. Absent param = ON, so
existing behavior is unchanged.

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

index.html

@@ -628,6 +628,20 @@
                         <span class="toggle-slider"></span>
                     </label>
                 </div>
+                <div class="setting-row">
+                    <span>
+                        Clear the field before typing
+                        <small class="setting-hint">
+                            Sends Ctrl+A + Del to empty the target field first.
+                            Disable for apps where Ctrl+A isn't "select all"
+                            (terminals, some remote desktops).
+                        </small>
+                    </span>
+                    <label class="toggle">
+                        <input type="checkbox" id="settingAutofillClear">
+                        <span class="toggle-slider"></span>
+                    </label>
+                </div>
                 <div id="settingAutofillHotkeysRow" style="margin-top:8px">
                     <div class="setting-row">
                         <span style="font-size:12px;color:var(--text-dim)">