style(x): unify tab headers and clean up Brain/Home UI
- Shared header treatment across Apps, Background tasks, Workspace,
Meetings, Brain: 24px/650 heading, no leading icons, no divider line,
centered 1120px column with fixed 34px/30px padding
- Apps-style page background on BG tasks, Workspace, Meetings
- Brain files view: monochrome then icon-free rows, inline note-count
meta line instead of pill chips, semibold item names, uniform row
heights, breadcrumb-only navigation, ghost quick actions
- Bases toolbar flush with table edge
- Meetings: full-width Coming up section
- Home: drop Connections/Ask-anything icon tiles, real Slack logo
- Sidebar: Apps above Background agents
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