tesks-ui: Navbar/sidebar redesign + settings accordion + shared design-token retune

Sidebar/navbar polish, plus the cross-cutting design-system foundation that the rest of the UI overhaul inherits (tracked here as the first screen tackled).

Shared foundations (global, consumed by all screen issues)

• Retune the radius scale in styles/_theme.scss — "everything is too rounded". Tighten control/card/menu radii (Linear-ish 4–6px) instead of 8–14px; keep pill only for genuinely pill things (badges, vote control).
• Introduce a shared page-container width token + primitive so every screen aligns to the same gutters/max-width (fixes integrations cards off to the left, kanban max-width gutters, projects/settings inconsistency).
• Tighten shared form/select/search primitives (_mixins.scss input-base, components/Select, components/Filters) since "search/filter/select look ugly" recurs on multiple screens.

Sidebar (components/Sidebar, WorkspaceSwitcher, CollapseToggle, AppShell)

• Workspace switcher + collapse icon look out of place together; the dropdown component itself is ugly — redesign both.
• Footer (logged-in user + role chip — the "linear badge" — + sign-out) is ugly; rework it, and move the user settings entry here.
• Add a small accordion so Settings expands into sub-items: Integrations / User settings / Workspace settings (sets up the Integrations + Settings issues).

One issue per screen per the agreed breakdown; commit to develop, ref this issue.