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/layout.css

@@ -0,0 +1,13 @@
+/* --- Layout --- */
+.feed-container {
+    width: 100%;
+    max-width: 600px;
+    padding: calc(20px + env(safe-area-inset-top)) 16px
+        calc(120px + env(safe-area-inset-bottom)) 16px;
+}
+
+@media (min-width: 768px) {
+    .feed-container {
+        padding: 40px 20px 150px 20px;
+    }
+}