fix: eslint findings

Remove dead code and unused imports, drop useless assignments, and tune
rules in eslint.config.js (underscore-prefixed unused names, intentional BOM/NBSP,
React Compiler rules from eslint-plugin-react-hooks 7 as warnings).

src/selftestBoot.ts

@@ -11,7 +11,7 @@ import type { StepResult } from "./lib/selfTestSteps";
 const WATCHDOG_MS = 120_000;
 
 export async function maybeRunSelfTest(): Promise<void> {
-  let reportPath: string | null = null;
+  let reportPath: string | null;
   try {
     reportPath = await invoke<string | null>("selftest_config");
   } catch {