Vendor backend and frontend into the monorepo

The submodule collapse was only half applied: .gitmodules was deleted but
backend/ and frontend/ were still recorded as gitlinks, so none of their
files were tracked. Replace the gitlinks with the real trees.

Also untrack .env (it carried placeholder secrets) in favour of a tracked
.env.example, drop the committed __pycache__, and narrow the blanket *.png
ignore that would have swallowed design assets.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/biome.json

@@ -0,0 +1,46 @@
+{
+  "$schema": "https://biomejs.dev/schemas/2.3.12/schema.json",
+  "assist": { "actions": { "source": { "organizeImports": "on" } } },
+  "files": {
+    "includes": [
+      "**",
+      "!**/dist/**/*",
+      "!**/node_modules/**/*",
+      "!**/src/routeTree.gen.ts",
+      "!**/src/client/**/*",
+      "!**/src/components/ui/**/*",
+      "!**/playwright-report",
+      "!**/playwright.config.ts"
+    ]
+  },
+  "linter": {
+    "enabled": true,
+    "rules": {
+      "recommended": true,
+      "suspicious": {
+        "noExplicitAny": "off",
+        "noArrayIndexKey": "off"
+      },
+      "style": {
+        "noNonNullAssertion": "off",
+        "noParameterAssign": "error",
+        "useSelfClosingElements": "error",
+        "noUselessElse": "error"
+      }
+    }
+  },
+  "formatter": {
+    "indentStyle": "space"
+  },
+  "javascript": {
+    "formatter": {
+      "quoteStyle": "double",
+      "semicolons": "asNeeded"
+    }
+  },
+  "css": {
+    "parser": {
+      "tailwindDirectives": true
+    }
+  }
+}