docs: add ARCHITECTURE.md and AGENTS.md, split CSS, fix justfile

- Add ARCHITECTURE.md with full system overview (data flow, modules, build)
- Add AGENTS.md as a guide for future AI agents working on this project
- Split monolithic styles.css into focused files under assets/:
  variables, animations, layout, navbar, header, cards, config,
  filters, utils, greet
- Fix justfile: replace `cargo run tauri dev/build` with `cargo tauri dev/build`

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/animations.css

@@ -0,0 +1,37 @@
+/* --- Animations --- */
+@keyframes float {
+    0%,
+    100% {
+        transform: translateY(0px);
+    }
+
+    50% {
+        transform: translateY(-15px);
+    }
+}
+
+@keyframes slideIn {
+    from {
+        opacity: 0;
+        transform: translateY(-10px);
+    }
+
+    to {
+        opacity: 1;
+        transform: translateY(0);
+    }
+}
+
+@keyframes pulse-subtle {
+    0% {
+        box-shadow: 0 0 0 0 rgba(74, 144, 226, 0.4);
+    }
+
+    70% {
+        box-shadow: 0 0 0 10px rgba(74, 144, 226, 0);
+    }
+
+    100% {
+        box-shadow: 0 0 0 0 rgba(74, 144, 226, 0);
+    }
+}