fix(ui): add reliable navbar unpin for pinned issues

[Codename-11]

Summary

• Adds a visible, accessible unpin action directly on each navbar pinned chip.
• Keeps pin caches and cross-tab navbar/sidebar state in sync after navbar unpin.
• Makes issue-detail pin controls target the navbar pin bucket again and labels navbar vs sidebar pin actions clearly.
• Adds role-addressable Combobox controls and updates E2E coverage for the current combobox/date-picker UI, including create → pin to navbar → unpin from navbar chip.
• Keeps DatePicker popovers inside the viewport when opened near the right/bottom edge.

Verification

• pnpm typecheck
• pnpm lint (passes; existing native-select warnings only)
• set -a; source /home/bailey/forge/.env; set +a; unset OPENAI_API_KEY; pnpm test → 1027 passed / 1 skipped
• set -a; source /home/bailey/forge/.env; set +a; unset OPENAI_API_KEY; pnpm build:app → Next build successful
• set -a; source /home/bailey/forge/.env; set +a; unset OPENAI_API_KEY; E2E_FORCE_BUILD=0 pnpm exec playwright test tests/e2e/issue-flow.spec.ts tests/e2e/mobile-smoke.spec.ts tests/e2e/sprints-roadmap.spec.ts --workers=1 → 10 passed

Closes AXI-89