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/header.css

@@ -0,0 +1,97 @@
+/* --- Feed Header & Controls --- */
+.feed-header {
+    display: flex;
+    justify-content: space-between;
+    align-items: flex-start;
+    margin-bottom: 24px;
+    flex-wrap: wrap;
+    gap: 16px;
+}
+
+.header-main {
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    width: 100%;
+}
+
+.header-main h1 {
+    margin: 0;
+    font-size: 1.5rem;
+}
+
+.header-actions {
+    display: flex;
+    gap: 8px;
+    align-items: center;
+}
+
+/* --- Mode Toggle (AI / RSS) --- */
+.mode-toggle {
+    display: flex;
+    background: rgba(255, 255, 255, 0.05);
+    border-radius: 12px;
+    padding: 3px;
+    border: 1px solid rgba(255, 255, 255, 0.08);
+}
+
+.toggle-btn {
+    border: none;
+    background: transparent;
+    color: #8e8e93;
+    padding: 8px 16px;
+    border-radius: 9px;
+    cursor: pointer;
+    font-size: 0.8rem;
+    font-weight: 700;
+    user-select: none;
+    min-height: 36px;
+}
+
+.toggle-btn.active {
+    background: var(--accent-color);
+    color: white;
+}
+
+/* --- Action Buttons (reload, settings) --- */
+.action-btn {
+    background: rgba(255, 255, 255, 0.05);
+    border: 1px solid rgba(255, 255, 255, 0.1);
+    border-radius: 12px;
+    padding: 12px;
+    cursor: pointer;
+    font-size: 1.1rem;
+    user-select: none;
+    min-width: 44px;
+    min-height: 44px;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+}
+
+/* --- AI Provider Switch --- */
+.provider-switch {
+    display: flex;
+    background: rgba(255, 255, 255, 0.05);
+    border-radius: 12px;
+    padding: 3px;
+    border: 1px solid rgba(255, 255, 255, 0.08);
+    margin: 0 10px;
+}
+
+.prov-btn {
+    border: none;
+    background: transparent;
+    color: var(--text-secondary);
+    padding: 6px 14px;
+    border-radius: 9px;
+    cursor: pointer;
+    font-size: 0.75rem;
+    font-weight: 700;
+    transition: all 0.2s ease;
+}
+
+.prov-btn.active {
+    background: var(--accent-color);
+    color: white;
+}