chore(web): add knip and remove unused npm dependencies

Drops highlight.js, lunr, autoprefixer, and @types/markdown-it —
all verified unreferenced (markdown-it ships its own types). Adds
knip.json configured for the templ/htmx architecture: main.ts and
reader/reader.ts entries, vitest tests as entry points, and an
ignore list for the ten modules whose exports are bound by templ
Alpine attributes — string bindings that static analysis cannot see.

knip.json

@@ -0,0 +1,26 @@
+{
+  "$schema": "./node_modules/knip/schema.json",
+  "entry": [
+    "web/src/**/*.test.ts"
+  ],
+  "project": [
+    "web/src/**/*.ts"
+  ],
+  "ignoreDependencies": [
+    "vite",
+    "vitest",
+    "htmx.org"
+  ],
+  "ignore": [
+    "web/src/dashboard.ts",
+    "web/src/stats.ts",
+    "web/src/header.ts",
+    "web/src/index.ts",
+    "web/src/login.ts",
+    "web/src/profile-modal.ts",
+    "web/src/profile.ts",
+    "web/src/bookshelf.ts",
+    "web/src/collections.ts",
+    "web/src/toast-error.ts"
+  ]
+}