Drop the floating fleet header; every Overview tab now renders its heading
inside the pane, under the tab strip, in the per-app detail .config-title
format (emoji + title + description) — matching the Migrate tab.
Introduces a small modular system so the area can't drift:
- OV_TAB_META is the single source of truth for each tab's icon/title/blurb.
- renderHeader(id) is the only thing that turns it into markup; renderTab()
prepends it for the string tabs and mountMigrate() injects it once for the
static Migrate pane. Body renderers now only ever produce the body.
Retires the now-dead floating-header plumbing: updateHeader(), the
ov-backups-active/ov-migrate-active hide toggles + CSS, and the .overview-header
rules. The tabbed interface owns the top padding the header used to provide.
Backups is the documented exception — it embeds the full BackupPage, which
supplies its own header.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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