refactor(ui): tidy two small clone groups (plugins grid, screen card setup)

Problem Statement

Two small duplicated blocks in the UI reported by fallow dupes:

1. packages/ui/src/views/PluginsOverviewView.vue:79-93 ↔ views/PluginsView.vue:72-86: the VRow/VCol grid that renders PluginCard per plugin (15 lines). Differences: the button label and the :device-id / event handlers passed to PluginCard.
2. packages/ui/src/components/AddScreenCard.vue:14-20 ↔ components/ScreenListCard.vue:15-20: identical setup (defineProps<{ deviceId }>, useScreensStore, useDeviceStore, device computed, useDeviceRenderTarget(device)).

Solution

1. Extract a PluginCardGrid.vue component taking plugins, optional deviceId, and emitting assignments-changed / deleted; use it from both views.
2. Extract a useDeviceRenderContext(deviceId) composable (in composeables/) returning { device, renderTarget, screensStore } and use it from both cards.

No behavior change; keep data-test-ids and run pnpm --filter ./packages/ui test:e2e.

Verification

• pnpm lint && pnpm type-check && pnpm test pass
• Run pnpm fallow:baseline in the same PR so the finding disappears from .fallow-baselines/, then confirm pnpm fallow:ci passes. Never add new findings to a baseline; see docs/agents/fallow.md.

Found by the initial fallow scan (pnpm fallow) on 2026-08-22 and baselined so CI stays green until this is fixed.