fix(web): stop throwing when right-clicking an uncached event

[posthog]

Summary

• Right-clicking a grid event whose id isn't in the react-query cache threw an uncaught Error("Selected event not found") and took down the grid — an uncaught throw in a core interaction (right-click to open the event menu), not a degraded state.
• The DOM id (data-event-id) can legitimately disagree with the cache: unsaved drafts render with draft.clientId (never cached), week transitions render from non-cache-backed placeholderData, and optimistically removed events can still be on screen for a frame.
• This was the only one of ~14 findEventInCache call sites that threw; every other caller treats a null result as normal.
• Fix: getDraftForEvent returns null on a miss and handleContextMenu bails out — same shape as the existing if (!draft) return guard. It also falls back to the current grid draft when the clicked id matches getGridDraftId(draft), so right-clicking an in-progress draft opens the menu instead of quietly doing nothing.

Simplicity

• Replaces the throwing getSelectedEvent with a single getDraftForEvent helper; the call site loses its separate editGridEventDraft(...) wrapping (now folded into the helper), so handleContextMenu gets simpler, not larger.

Automated validation

• Did not run the browser app. The symptom is an uncaught exception (not a visual/layout state); it's reproduced and pinned by a unit test that right-clicks a card whose id is absent from the cache and asserts no throw and no menu.

Independent review

• Self-reviewed the diff. The fallback returns the store's live GridEventDraft directly (the same object startGridDraft/gridEventDraftToGridEvent already consume elsewhere), so no new shape is introduced.

Test plan

• bun test src/components/ContextMenu/contextMenuLayering.test.tsx (3 pass, incl. the new "ignores a right-click on an event that isn't in the cache")
• bun run type-check
• biome check on the changed files

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