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>

justfile

@@ -0,0 +1,12 @@
+default:
+    just --list
+
+run:
+    cargo tauri dev
+
+build:
+    cargo tauri build --apk
+
+check:
+    cargo check
+    cargo clippy