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/greet.css

@@ -0,0 +1,36 @@
+/* --- Greet Page --- */
+.robot-container {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    cursor: pointer;
+    margin: 40px auto;
+    width: 100%;
+    user-select: none;
+}
+
+.robot-icon {
+    font-size: 5rem;
+    filter: drop-shadow(0 0 20px rgba(74, 144, 226, 0.3));
+    animation: float 3s ease-in-out infinite;
+}
+
+.bubble {
+    margin-top: 20px;
+    padding: 16px 24px;
+    background: rgba(30, 30, 46, 0.8);
+    border-radius: 20px;
+    border: 1px solid rgba(255, 173, 210, 0.3);
+    color: #ffffff;
+    font-weight: 600;
+    font-size: 1rem;
+    text-align: center;
+    max-width: 90%;
+}
+
+@supports (backdrop-filter: blur(12px)) {
+    .bubble {
+        backdrop-filter: blur(12px);
+    }
+}