feat(dashboard): phase B drag-resize grid (Gridstack) replacing masonry

Milestone 72 phase B — Grafana-style drag-resize grid for dashboard widgets:
- DashboardWidget: replace `position` with a 12-col grid placement
  (grid_x/grid_y/grid_w/grid_h). Migration 0021 backfills the old position
  order into a 3-up grid (4 cols x 4 cells each) and drops position.
- View + share render a static CSS grid from x/y/w/h: fixed cell height
  (h * 70px) with the body scrolling, so the arranged layout is what's shown;
  collapses to a single column under 820px.
- Edit view: Gridstack.js 12.6.0 (vanilla, CDN, pinned) — drag the title bar
  to move, drag a corner/edge to resize; every change autosaves to a new
  /d/<id>/edit/layout endpoint. Replaces the SortableJS position reorder.
- add_widget appends at the bottom-left; remove no longer renumbers.
  _get_widgets now orders by grid position (drives DOM + mobile fallback order).

Note: Gridstack drag-resize is browser-only, so CI can't exercise it — needs an
operator visual check of the edit experience.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/templates/dashboard/index.html

@@ -62,14 +62,16 @@
 
 {# ── Widget grid ───────────────────────────────────────────────────────────── #}
 {% if widgets %}
-<div style="column-width:380px;column-count:3;column-gap:1rem;">
+<div class="dash-grid">
   {% for item in widgets %}
-  <div class="card ping-card" style="break-inside:avoid;margin-bottom:1rem;">
+  {% set gw = item.db %}
+  <div class="card dash-cell"
+       style="grid-column:{{ gw.grid_x + 1 }} / span {{ gw.grid_w }};grid-row:{{ gw.grid_y + 1 }} / span {{ gw.grid_h }};">
     <div class="widget-header">
-      <span class="widget-title">{{ item.title }}</span>
+      <span class="widget-title" title="{{ item.title }}">{{ item.title }}</span>
       <a href="{{ item.defn.detail_url }}" class="widget-link">Details →</a>
     </div>
-    <div id="widget-{{ item.db.id }}"
+    <div class="dash-cell-body" id="widget-{{ gw.id }}"
          hx-get="{{ item.hx_url }}"
          hx-trigger="load{% if item.defn.poll %}, every {{ poll_interval }}s{% endif %}"
          hx-swap="innerHTML">