fix(ui): prevent anchored overlays from clipping - #2029

Summary

• add one viewport-aware anchored popover hook that portals overlays out of clipping ancestors, flips at viewport edges, clamps both axes, and follows nested scrolling
• migrate the shared Select plus active menus and panels across market-adjacent settings, chat input, FlowChat header, navigation, shell, dispatch, profile, welcome, session files, and tool timeout surfaces
• preserve the explicit inline Select mode used by the layout-expanding Agents surface and preserve surface-specific dropdown styling after portalization
• add geometry and clipping regression coverage, including assertions that dispatch and permission menus anchor to their actual trigger buttons
• make the existing ExternalSources confirmation test target its dialog instead of relying on body DOM order after portal hosts are reused

Verification

• pnpm run type-check:web
• pnpm run lint:web
• pnpm --dir src/web-ui run test:run (408 files, 2772 tests)
• pnpm run build:web
• pnpm run theme:color-audit:all
• pnpm run check:repo-hygiene