fix(#840): map popup 'Show Neighbors' link does nothing on iOS Safari (#841)

Closes #840

## What
Switch the map-popup "Show Neighbors" link from `<a href="#">` to `<a
href="javascript:void(0)" role="button">` so iOS Safari doesn't navigate
when the document-level click delegation fails to fire.

## Why
On iOS Safari, when a user taps the link inside a Leaflet popup:
- The document-level click delegation at `public/map.js:927` calls
`e.preventDefault()` and triggers `selectReferenceNode`.
- BUT inside a Leaflet popup, `L.DomEvent.disableClickPropagation()` is
internally applied to popup content — on iOS Safari the click sometimes
doesn't bubble to `document`.
- When that happens, the browser's default `<a href="#">` action runs:
  - hash becomes empty (`#`)
- `navigate()` in `app.js:458` sees empty hash → defaults to `'packets'`
- map page is destroyed mid-tap → user perceives "nothing happened" (or
a brief flash if they back-button)

`href="javascript:void(0)"` removes the navigation fall-through
entirely. The `role="button"` keeps a11y semantics, `cursor:pointer`
keeps the visual cue.

## Tested
- Headless Chromium desktop + iPhone 13 emulation: tap fires
`/api/nodes/{pk}/neighbors?min_count=3`, marker count drops from 441 →
44, `#mcNeighbors` checkbox toggles on, URL stays on `/#/map`. Same as
before.
- Frontend helpers: 556/0
- Real iOS Safari fix verification needs a physical-device test
post-deploy

## Out of scope (follow-up)
- Same `<a href="#">` pattern exists for the topright "Close route"
control at `public/map.js:389` — uses `L.DomEvent.preventDefault` so
should work, but worth auditing if the symptom recurs.

Co-authored-by: Kpa-clawbot <agent@corescope.local>

public/map.js

@@ -965,7 +965,7 @@
         </dl>
         <div style="margin-top:8px;clear:both;">
           <a href="#/nodes/${node.public_key}" style="color:var(--accent);font-size:12px;">View Node →</a>
-          ${node.public_key ? ` · <a href="#" data-show-neighbors data-pubkey="${escapeHtml(node.public_key)}" data-name="${escapeHtml(node.name || 'Unknown')}" style="color:var(--accent);font-size:12px;">Show Neighbors</a>` : ''}
+          ${node.public_key ? ` · <a href="javascript:void(0)" role="button" data-show-neighbors data-pubkey="${escapeHtml(node.public_key)}" data-name="${escapeHtml(node.name || 'Unknown')}" style="color:var(--accent);font-size:12px;cursor:pointer;">Show Neighbors</a>` : ''}
         </div>
       </div>`;
   }