Fix judge findings on the landed family redesign

[alexeygrigorev]

Fixes the five concrete findings from the family redesign judge round against the merged PR #31 tree. One commit per finding.

1. Mobile admin-keys table overflow (family-breaking)

   • templates/mailing/operator/admin_api_keys.html: the Keys table now uses the established mobile stacking pattern copied from client_detail.html — table-wrap stack-table wrapper plus data-label on every cell, so rows render as labelled rows at phone width instead of a clipped grid.
   • Commit: "Stack the admin API keys table into labelled rows on mobile"

2. Revoke rendered as a second accent-filled primary (family-breaking, one-primary-per-view)

   • Same file: the revoke submit button now carries class="danger" — the outlined destructive control already defined in static/mailing/css/app.css (.danger vocabulary) and used the same way in client_detail.html.
   • Commit: "Render the admin key revoke action as the outlined danger control"

3. Keys table sat outside the standard list container (polish)

   • The Keys list now sits in the same bordered banded container as every other list: section panel + panel-header muted band with a key count, copied from the campaigns/client list templates.
   • Commit: "Move the admin API keys list into the banded panel container"

4. Dashboard summary strip missing vertical hairline cell dividers (polish)

   • Root cause was not missing markup classes (dashboard.html already had stats-grid/stat): each strip cell painted an opaque background, and because cells carry margin-right/bottom: -1px so wrapped rows stay divided, each next cell's background painted exactly 1px over the previous cell's border hairline. Verified in-browser: computed border-right: 1px solid present, adjacent cell rects overlap by exactly 1px, no divider pixels rendered.
   • Fix in static/mailing/css/app.css: strip cells (.stat, .meta-grid > *) stay transparent and the container paints the surface/page color, so the hairlines show through the intentional 1px overlap. Rendered color is unchanged; dividers now render on the home summary strip, the processing strip, and detail-page strips/meta grids.
   • Commit: "Let strip hairlines render through the negative-margin cell overlap"

5. Raw internal "Django admin" link on the operator-facing page (polish)

   • Dropped from the admin API keys page header per the operator-page copy standard.
   • Commit: "Drop the Django admin link from the operator admin keys page"

Verification: uv run pytest 754 passed (unchanged baseline), uv run ruff check . clean, plus pixel-level captures of the fixed home strip (divider columns present) and the Revoke button (outlined, not accent-filled).

Out of scope, untouched: icon geometry, content container width, account menu depth, mobile header action layout.