feat(ui): add search clear button

Search now exposes a small icon-only clear button whenever a query is present.
Clicking it clears the term in one step and returns focus to the input so users
can immediately type a replacement.

The button uses the existing topbar styling language and a compact circled-x
icon alongside the keyboard hint.

Test Plan:
- git diff --check

Refs: user redesign nitpick about one-click search clearing

crates/lanspread-tauri-deno-ts/src/components/topbar/SearchField.tsx

@@ -49,6 +49,19 @@ export const SearchField = ({ value, onChange }: Props) => {
                 }}
                 spellCheck={false}
             />
+            {value && (
+                <button
+                    className="search-clear"
+                    type="button"
+                    aria-label="Clear search"
+                    onClick={() => {
+                        onChange('');
+                        inputRef.current?.focus();
+                    }}
+                >
+                    <Icon.clearCircle />
+                </button>
+            )}
             <span className="search-kbd">/</span>
         </div>
     );