feat(flowchart): visual polish — selection ring, hover, resize grip (C16)

Adds the visual feedback that was missing from the C10 resize + C2
selection commits. The canvas previously set the .selected class but
no CSS rule styled it, so a selected node looked identical to an
unselected one. Same for hover and the resize handle.

CSS additions (src/flowchart-generator.html):

  - .flowchart-node: cursor:grab + 0.12s filter transition for the
    hover tint (drop-shadow blue glow).
  - .flowchart-node:hover .flowchart-node-shape: blue glow.
  - .flowchart-node.selected .flowchart-node-shape: 2.25px blue
    stroke — replaces the (previously invisible) .selected class.
  - .flowchart-edge:hover and .flowchart-edge.selected: blue
    stroke + 2.5px width for the same reason.
  - .flowchart-resize-handle-hit: 24px transparent rect for an
    enlarged hit area (the visible grip stays 10px).
  - .flowchart-resize-handle-grip: blue square with stroke, styled
    via CSS instead of inline fill/stroke (which were overriding
    the rest of the file's theme).
  - .flowchart-edge-label-bg: opaque white pill behind edge labels,
    so labels stay readable when an edge curves under them.
  - .fc-modal / .fc-modal-overlay: fade + scale-in keyframes
    (0.15s ease-out) so the promptInline / confirmInline modals
    animate in instead of popping in instantly.

Canvas (src/flowchart/flowchart-canvas.js): the resize handle now
emits two rects — a 24px transparent hit area (data-resize-node
attribute preserved) and a smaller 10px visible grip styled via
.flowchart-resize-handle-grip. Cursor stays nwse-resize via CSS.

Tests: no new tests (visual); existing 14 canvas tests still pass.
1376 total.

Amit Haridas

src/flowchart-generator.html

@@ -185,6 +185,77 @@
       .flowchart-edge {
         stroke: #1f2328 !important;
       }
+      /* v4.13.0 — visual polish. Adds a visible selection ring (was
+         previously only the .selected class without a distinct colour),
+         a hover tint, an enlarged resize-handle hit area, a smoother
+         modal dialog animation, and a proper opaque background behind
+         edge labels so they stay readable across curved connectors. */
+      .flowchart-node {
+        cursor: grab;
+        transition: filter 0.12s ease-out;
+      }
+      .flowchart-node:hover .flowchart-node-shape {
+        filter: drop-shadow(0 0 2px rgba(37, 99, 235, 0.45));
+      }
+      .flowchart-node.selected .flowchart-node-shape {
+        stroke: #2563eb !important;
+        stroke-width: 2.25;
+      }
+      .flowchart-edge {
+        cursor: pointer;
+      }
+      .flowchart-edge:hover {
+        stroke: #2563eb !important;
+      }
+      .flowchart-edge.selected {
+        stroke: #2563eb !important;
+        stroke-width: 2.5;
+      }
+      /* Resize handle — larger invisible hit area + visible square. */
+      .flowchart-resize-handle {
+        cursor: nwse-resize;
+      }
+      .flowchart-resize-handle-hit {
+        fill: transparent;
+        stroke: none;
+      }
+      .flowchart-resize-handle-grip {
+        fill: #2563eb;
+        stroke: #1e40af;
+        stroke-width: 1;
+      }
+      /* Edge label background — keeps labels legible across curves by
+         giving them an opaque white pill behind the text. */
+      .flowchart-edge-label-bg {
+        fill: #ffffff;
+        stroke: #d1d5db;
+        stroke-width: 0.5;
+      }
+      /* Modal dialog — fade + scale-in entrance. */
+      .fc-modal {
+        animation: fc-modal-in 0.15s ease-out;
+      }
+      @keyframes fc-modal-in {
+        from {
+          opacity: 0;
+          transform: translate(-50%, -50%) scale(0.96);
+        }
+        to {
+          opacity: 1;
+          transform: translate(-50%, -50%) scale(1);
+        }
+      }
+      .fc-modal-overlay {
+        animation: fc-overlay-in 0.12s ease-out;
+      }
+      @keyframes fc-overlay-in {
+        from {
+          opacity: 0;
+        }
+        to {
+          opacity: 1;
+        }
+      }
       /* v4.10.0 — preview-render info card explaining the layout */
       .fc-preview-render-note {
         font-size: 0.75rem;