fix(co4e): section actions are text again, and "+ Mới" says it did something

* "+ Mới" / "Quản lý skill…" were full buttons, ~40px tall each — the
    loudest thing in a 230px sidebar, and three rows of height spent on
    them. The wireframe writes these as small accent text beside the
    heading; they are now 17px.

  * "+ Mới" looked broken. It was not: it replaces the canvas with a fresh
    empty flow, verified by loading a saved 3-node flow and pressing it
    (3 nodes → 0). But pressing it while the canvas ALREADY held an empty
    untitled flow produced an identical empty untitled flow — right, and
    invisible. It now focuses and selects the name field, and says
    "Flow mới — đặt tên rồi kéo agent vào canvas" in the status bar, so the
    press always has a visible result.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

theme.py

@@ -430,6 +430,16 @@ QTabBar#viewTabs::tab:hover { background: $hover; color: $text; }
 QTabBar#viewTabs::tab:selected { background: $active; color: $text; font-weight: 600; }
 /* Co4E sidebar section headings — same quiet caps as the rail's RECENTS, so
    "which list am I looking at" is answered on screen, not in a tooltip. */
+/* The action beside a Co4E section heading ("+ Mới", "Quản lý skill…"). The
+   wireframe writes these as small accent text; as full buttons they were the
+   loudest thing in the sidebar and each cost a row of height. */
+QPushButton#co4eSectionAction {
+    background: transparent; border: none; color: $accent;
+    font-size: 11px; font-weight: 600; padding: 1px 4px;
+    border-radius: ${radius_sm}px; qproperty-iconSize: 11px 11px;
+}
+QPushButton#co4eSectionAction:hover { background: $hover; color: $accent; }
+QPushButton#co4eSectionAction:pressed { background: $active; }
 QPushButton#co4eSectionHdr {
     color: $text_faint; font-size: 11px; font-weight: 700; letter-spacing: 0.5px;
     background: transparent; border: none; text-align: left; padding: 2px 0;