Perform dead code analysis with Knip (#2575)

* Install Knip

* Clarify an import that was confusing Knip

* Fix issues detected by Knip

Including cleaning up some unused code and dependencies, using a React hook that we unintentionally stopped using, and also adding some previously undeclared dependencies.

* Run dead code analysis in lint script and CI

---------

Co-authored-by: Timo <toger5@hotmail.de>

knip.ts

@@ -0,0 +1,30 @@
+import { KnipConfig } from "knip";
+
+export default {
+  entry: ["src/main.tsx", "i18next-parser.config.ts"],
+  ignoreBinaries: [
+    // This is deprecated, so Knip doesn't actually recognize it as a globally
+    // installed binary. TODO We should switch to Compose v2:
+    // https://docs.docker.com/compose/migrate/
+    "docker-compose",
+  ],
+  ignoreDependencies: [
+    // Used in CSS
+    "normalize.css",
+    // Used for its global type declarations
+    "@types/grecaptcha",
+    // Because we use matrix-js-sdk as a Git dependency rather than consuming
+    // the proper release artifacts, and also import directly from src/, we're
+    // forced to re-install some of the types that it depends on even though
+    // these look unused to Knip
+    "@types/content-type",
+    "@types/sdp-transform",
+    "@types/uuid",
+    // We obviously use this, but if the package has been linked with yarn link,
+    // then Knip will flag it as a false positive
+    // https://github.com/webpro-nl/knip/issues/766
+    "@vector-im/compound-web",
+    "matrix-widget-api",
+  ],
+  ignoreExportsUsedInFile: true,
+} satisfies KnipConfig;