web: stage partial input — replay tool-input-partial before final tool input

[cliffhall]

Summary

Add a "Stage partial input" control to the Apps screen for testing progressive-render widgets: each press snapshots the current form values; on "Open App", the snapshots are replayed to the view as ui/notifications/tool-input-partial fragments before the final tool-input.

Why

Widgets that render progressively from partial tool input can't be exercised in the inspector at all today — the host only ever sends the final input.

Reference implementation (PR #1510)

Re-implement informed by these changes at 33fac3f:

• clients/web/src/components/elements/AppRenderer/AppRenderer.tsx — partialInputs prop, pendingPartialsRef, flushPending() sending bridge.sendToolInputPartial() before tool-input
• clients/web/src/components/screens/AppsScreen/AppsScreen.tsx — partialStages state, handleStagePartialInput(), PartialStageControls/StagePartialButton/PartialStageCount subcomponents, "Clear staged"

Depends on

• CSP enforcement + resource-error surfacing in the bridge factory (same files; land first)

Wave 2 lane — sequential with the other AppRenderer/AppsScreen issues.

Notes

• Mantine rules for the new controls: .withProps() constants, theme variants, no inline styles.
• Coverage gate ≥90 on all four dimensions.

Part of the PR #1510 decomposition (see tracking issue).