Consistent page shell: one page header component, shared gutters/measures, breadcrumbs, sticky toolbars

Part of PAR-357.

Problem

• Headers jump between pages. Issues and Board use measure="data" (full bleed, 8–16px gutter). Projects, Teams, Settings, Integrations and Usage use page (1280px centred, 12–32px gutter). Issue detail uses prose (72ch centred) (DS/Surface/Surface.module.scss:1-11). Switching between Issues and Projects visibly shifts the header (x=331 vs x=355).
• Every page reinvents its header: "Issues (i) 349 issues…", "Board PAR — Paradaux", "Projects Paradaux", "← Projects / Hibernia Economy [planned]", "← Releases / Release: … Business".
• The top bar is mostly empty. There are no breadcrumbs, and the page title only shows on mobile.
• Toolbars scroll away with the content.

Do

• Add a PageHeader with a breadcrumb in the top bar, the title, optional tabs, and right-aligned actions (view toggle, display, filter, new). Every route uses it.
• Unify gutters: list and board stay full-bleed with a 16–24px gutter; detail and settings use the same left edge with a constrained content column.
• Keep header and filter toolbars sticky on scroll, with a hairline border once scrolled.
• Use one consistent empty-state component (icon, line of copy, primary action).