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/variables.css

@@ -0,0 +1,40 @@
+/* --- Root Variables & Reset --- */
+:root {
+    --bg-color: #0f0f13;
+    --card-bg: rgba(255, 255, 255, 0.03);
+    --card-border: rgba(255, 255, 255, 0.08);
+    --text-primary: #e2e2e7;
+    --text-secondary: #a0a0b0;
+    --accent-color: #4a90e2;
+    --nav-bg: rgba(30, 30, 46, 0.95);
+    --danger: #ef4444;
+}
+
+* {
+    box-sizing: border-box;
+    -webkit-tap-highlight-color: transparent;
+}
+
+body {
+    background-color: var(--bg-color);
+    color: var(--text-primary);
+    font-family:
+        "Inter",
+        -apple-system,
+        BlinkMacSystemFont,
+        "Segoe UI",
+        Roboto,
+        sans-serif;
+    margin: 0;
+    line-height: 1.5;
+    display: flex;
+    justify-content: center;
+    min-height: 100vh;
+    overflow-x: hidden;
+    -webkit-tap-highlight-color: transparent;
+}
+
+html {
+    scroll-behavior: smooth;
+    overflow-x: hidden;
+}