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/lib/selfTestE2e.ts

@@ -102,7 +102,6 @@ export async function runE2eSteps(log: StepLog, reportPath: string): Promise<voi
       },
     );
 
-  let inv!: Invoice;
   const issued = await log.run("e2e: issue and archive", async () => {
     const outcome = await issueAndArchive(input, prefs, deps);
     invoice = outcome.invoice;
@@ -112,7 +111,7 @@ export async function runE2eSteps(log: StepLog, reportPath: string): Promise<voi
     };
   });
   if (!issued.ok || !invoice) return;
-  inv = invoice as Invoice;
+  const inv = invoice as Invoice;
 
   await log.run("e2e: archive status", async () => {
     const st = await api.archiveStatus(inv.id);