feat(map): two-click selection model with precise center dot

Replaces 'every click selects something' with a deliberate two-click
flow:
- First click drops marker (existing circle plus new precise center dot)
  and opens place panel
- Second click INSIDE the marker circle opens the radial menu
- Second click OUTSIDE the circle deselects without selecting the new
  spot — requires another click to select

The 4px filled center dot at exact click coordinates gives precise
visual feedback for GPS-coord readout. The existing circle's radius
defines the same-spot tolerance, visually showing the radial-trigger
hit area.

Right-click radial unchanged. Search-dropdown selection drops a marker
for consistency.

src/index.css

@@ -259,6 +259,7 @@ body {
 
 /* ═══ PREVIEW PIN (selected but not committed) ═══ */
 .navi-pin-preview {
+  position: relative;
   width: 28px;
   height: 28px;
   border-radius: 50%;
@@ -267,6 +268,17 @@ body {
   box-shadow: 0 0 0 3px var(--accent-muted), var(--shadow);
   pointer-events: none;
 }
+.navi-pin-center-dot {
+  position: absolute;
+  left: 50%;
+  top: 50%;
+  transform: translate(-50%, -50%);
+  width: 4px;
+  height: 4px;
+  border-radius: 50%;
+  background: var(--accent);
+  pointer-events: none;
+}
 
 /* ═══ PLACE DETAIL PANEL ═══ */
 .navi-place-detail {