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/cards.css

@@ -0,0 +1,50 @@
+/* --- Post & News Cards --- */
+.post-card {
+    background: var(--card-bg);
+    border: 1px solid var(--card-border);
+    border-radius: 18px;
+    padding: 16px;
+    margin-bottom: 16px;
+    word-wrap: break-word;
+}
+
+.post-author {
+    color: var(--accent-color);
+    font-weight: 800;
+    font-size: 0.85rem;
+}
+
+.post-meta {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+}
+
+.post-category-badge {
+    font-size: 0.7rem;
+    font-weight: 700;
+    color: var(--accent-color);
+    background: rgba(74, 144, 226, 0.12);
+    border: 1px solid rgba(74, 144, 226, 0.25);
+    padding: 2px 8px;
+    border-radius: 10px;
+}
+
+/* --- Markdown Content --- */
+.markdown-body {
+    font-size: 0.95rem;
+    color: var(--text-primary);
+    line-height: 1.6;
+}
+
+.markdown-body h3 {
+    margin-top: 0;
+    margin-bottom: 10px;
+    font-size: 1.15rem;
+    color: #ffffff;
+    font-weight: 700;
+}
+
+.markdown-body p {
+    margin: 0;
+}