The /system page speaks the home page's language #9786

Bring /system (src/web/pages/system.ts) into line with the home page, the same way Settings › Backups did in #9754: headings over rows, not boxed cards.

Done looks like:

• [x] Every card( in src/web/pages/system.ts is gone. Each part is a <section class="settings-block"> with an <h2>, and its lines are row()s in a .list. When a part has nothing, it says so in one plain sentence.
• [x] Live regions keep their ids and data-live attributes (for example sys-disks), so the page still updates in place.
• [x] A test (for example test/system-parts.test.ts) renders the page as an admin and checks for one <h2> per part and no .card. It fails on main.
• [x] docs/design/areas/ records the shape in a short paragraph. Add no new component class unless one is needed, and if one is, put it in the area doc.
• [x] bun scripts/design-lint.ts is clean and bun test passes.

Files: src/web/pages/system.ts, src/web/style/*.ts if a rule is needed, docs/design/areas/system.md (new) or settings.md, test/. Risks: /system is polled live, so a renamed id silently stops the updates.

Reviewed pull #9791 at its head: ticked Every card( in src/web/pages/system.ts is gone. Each part is a <section class="settings-block"> with an <h2>, and its lines are row()s in a .list. When a part has nothing, it says so in one plain sentence.; Live regions keep their ids and data-live attributes (for example sys-disks), so the page still updates in place.; A test (for example test/system-parts.test.ts) renders the page as an admin and checks for one <h2> per part and no .card. It fails on main.; docs/design/areas/ records the shape in a short paragraph. Add no new component class unless one is needed, and if one is, put it in the area doc.; bun scripts/design-lint.ts is clean and bun test passes..