Fix giant Select-All checkbox (class collision) + disable UI text selection

- Select-All checkbox rendered huge when not-all-selected because its empty-state
  modifier class 'empty' collided with the .empty full-panel placeholder rule
  (flex:1; padding:30px). Renamed the modifier to 'off'.
- Disable text selection on UI chrome (native-app feel); keep it enabled for
  inputs, the code editor/viewer, terminal output, log, blame/diff, AI summary.

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

src/App.tsx

@@ -1181,8 +1181,7 @@ function Workbench({ info, session, light, toggleTheme, lang, setLang, zoom, set
                 </div>
               )}
               <div className="chhead">
-                <span className={"chk" + (allChecked ? "" : " empty")} onClick={toggleAll} title={allChecked ? t("Deselect all") : t("Select all")}
-                  style={{ width: 16, height: 16, minWidth: 16, flex: "0 0 16px", boxSizing: "border-box", alignSelf: "center" }}>{allChecked ? I.check : null}</span>
+                <span className={"chk" + (allChecked ? "" : " off")} onClick={toggleAll} title={allChecked ? t("Deselect all") : t("Select all")}>{allChecked ? I.check : null}</span>
                 <span>{t("{a} of {b} selected", { a: checked.size, b: view.length })}</span>
                 <button className={"scanbtn" + ((busy || scanning) ? " spinning" : "")} style={{ marginLeft: "auto" }} onClick={() => refresh()} disabled={busy || scanning} title={t("Re-read open files (p4 opened)")}>
                   {I.sync}<span>{scanning ? t("Scanning…") : t("Refresh")}</span>