U2 · App shell: sidebar with Pinned, breadcrumb top bar, mobile tab bar + drawer, layout-matched loading skeletons

UI foundation step 2. Depends on U1. Implements PAR-375 (as revised: no Inbox, Drafts or Favourites; a customisable Pinned section), PAR-394, PAR-389, PAR-362 (loading) and the shell half of PAR-387 (mobile).

Reference: the stories "Screens / Sidebar — editing pinned", "Screens / Navigation drawer (mobile)" and "Loading / *", plus Shell.tsx and LoadingScreens.tsx in the prototype.

Do

• Sidebar:
  • workspace switcher with the taskl.ie mark, search and new-issue icon buttons
  • My issues
  • Pinned (edit mode: reorder, unpin, "Pin something…")
  • Workspace: Projects, Deployments, Analytics
  • Teams (collapsible, each with Triage and Issues)
  • footer: avatar menu, invite, help, settings
• Pins: persist them per user. Until a backend endpoint exists, use local storage and leave a TODO that cites PAR-417 (workspaces domain).
• Top bar: a breadcrumb of the current location, with per-page actions on the right.
• Mobile (≤760px): tab bar (My issues, Issues, Triage, Projects, Search), floating create button, sidebar as a drawer, top bar showing only the current page.
• Loading:
  • Delete or slim app/(app)/loading.tsx so the shell never unmounts.
  • Add per-route loading.tsx skeletons (list, detail, board, projects, settings) using the DS Skeleton.
  • Add a thin accent progress bar for route transitions.
• Remove the Board nav item (it's a view toggle on Issues), Releases, Usage (it moves to settings) and the old Settings group (settings get their own layout in PAR-384).

Acceptance

• No full-screen loading takeover anywhere. Verify on dev with the test account.
• Mobile works at 390×844, and keyboard and a11y pass.
• Lint, typecheck, tests and stories pass, with screenshots in the PR.