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/navbar.css

@@ -0,0 +1,48 @@
+/* --- Navigation Dock --- */
+.navbar-dock {
+    position: fixed;
+    bottom: calc(20px + env(safe-area-inset-bottom));
+    left: 50%;
+    transform: translateX(-50%);
+    background-color: var(--nav-bg);
+    padding: 12px 24px;
+    border-radius: 50px;
+    border: 1px solid rgba(255, 255, 255, 0.1);
+    display: flex;
+    gap: 20px;
+    box-shadow: 0 10px 25px rgba(0, 0, 0, 0.3);
+    z-index: 1000;
+    width: max-content;
+    max-width: 90%;
+}
+
+@supports (backdrop-filter: blur(12px)) or (-webkit-backdrop-filter: blur(12px)) {
+    .navbar-dock {
+        background-color: rgba(30, 30, 46, 0.85);
+        backdrop-filter: blur(12px);
+        -webkit-backdrop-filter: blur(12px);
+    }
+}
+
+.nav-link {
+    color: var(--text-secondary);
+    text-decoration: none;
+    font-weight: 600;
+    font-size: 0.9rem;
+    transition: all 0.3s cubic-bezier(0.175, 0.885, 0.32, 1.275);
+    padding: 10px 12px;
+    user-select: none;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    min-height: 44px;
+}
+
+.nav-link:hover {
+    color: #ffffff;
+    transform: translateY(-3px);
+}
+
+.nav-link.active {
+    color: var(--accent-color);
+}