Bring the Open Wall gallery into the new design

[openipc-ai]

Part of #206 (Phase 2). Depends on nothing.

Why

Phase 1 resurrected /open-wall and put a live mosaic on the homepage, but the gallery itself (app/views/snapshots/index, show, oneday and the _icon, _slide, _snapshot partials) still renders in the old container, styled by pages/_openwall.scss, and test/controllers/legacy_pages_test.rb:70-75 pins it there alongside the legacy pages. It was not on June's list; it is on this one because it is the most visited page after the homepage and the one the homepage mosaic sends people to.

What to change

• index, show and oneday set content_for :fullwidth; the grid, the camera page and the one-day strip rebuilt on the new tokens; pages/_openwall.scss rewritten.
• Nothing on the upload path changes: POST /snapshots, SnapshotsController#create, the jobs, the image URLs and the admin views are out of scope. The rate-limit and blacklist tests keep passing untouched.
• /open-wall moves from legacy_pages_test.rb to relaunch_pages_test.rb.
• Screenshots use a presentable scene, not a bench.

Done means

• The three views render full-bleed in three locales on a database with snapshots and on an empty one (dev has no cameras).
• The upload API test and the snapshot model tests are green without edits.
• Screenshots before any PR.