Follow-up from #1532. Previously `body.dashboard-shell` / `body.home-shell`
carried the 1.5em horizontal gutter, and full-width chrome broke out of it
with negative margins (`.dashboard-chrome { margin: 0 -1.5em }`, the
`#server-warnings { margin: 0 -1.5em }` override).
Invert it: `<body>` is now full-bleed, and the padded page content lives in
a single inner `.page-content` wrapper that carries the gutter. The
server-warnings banner, the sticky tab strip, and the footer then span the
full width for free — so the `-1.5em` breakout hacks are gone.
- `.page-content { padding: 0 1.5em }` is a shared primitive in common.css
(the dashboard + H0M3 both opt in by wrapping their content; FL0W / L0GS /
ST4TS / S3TT1NGS stay full-bleed with their own `.<page>-main` padding).
- dashboard.html / index.html wrap their content in `.page-content`.
- dashboard.css / home.css: drop the body horizontal gutter (keep
`padding-bottom` for foot breathing room); `.dashboard-chrome` margin
`0 -1.5em 1em` → `0 0 1em`.
- common.css: drop the `#server-warnings` breakout override.
Behaviour/visual-neutral by intent (content gutter unchanged; chrome +
banner already rendered full-width via the old breakout). Worth a gui
screenshot-diff to confirm — esp. the now-full-width footer divider.
Last fold-in of #1464 step 2.
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