ref(snapshots): Make the snapshots toolbar presentational and de-duplicate

[mtopo27]

First of a 3-PR stack splitting #115893. PR 1 of 3 — base: `master`.

Refactors the preprod snapshots toolbar so it can be rendered in isolation, without touching production behavior.

What changes

• Extracts the toolbar components into a dedicated `snapshotsToolbar.tsx` module and makes them purely presentational.
• `DiffModeToggle` now takes a `showSplit` prop instead of calling `useBreakpoints()` itself. The view-mode / diff-mode toggles no longer call `useOrganization()` / `trackAnalytics` — that wiring moves up into `SnapshotMainContent`, which passes `breakpoints.sm` and fires analytics from its `onChange` handlers.
• `snapshotMainContent.tsx` imports the shared components instead of defining its own copies, removing the duplication.
• Drops the now-unused `export` on `TRANSPARENT_COLOR` in `diffImageDisplay.tsx` (still used internally; no external importer remains after the de-dupe).

Behavior

No production behavior change intended. The old `DiffModeToggle` did `const breakpoints = useBreakpoints(); breakpoints.sm ? …`, which is exactly what `showSplit={breakpoints.sm}` now passes.

Follow-ups in the stack

• PR 2: snapshot-harness plumbing to render real design-system components under SSR.
• PR 3: the actual toolbar snapshot tests.

[sentry]

Bug: The handleOpenSnapshot function calls onViewModeChange directly, bypassing the handleViewModeChange wrapper and skipping analytics tracking when a snapshot is opened from the list view.
_{Severity: MEDIUM}

Suggested Fix

In handleOpenSnapshot, replace the direct call to onViewModeChange('single') with a call to the wrapper function handleViewModeChange('single'). This will ensure the analytics event is correctly tracked when a user opens a snapshot.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: static/app/views/preprod/snapshots/main/snapshotMainContent.tsx#L160

Potential issue: When a user opens a snapshot from the list view, the
`handleOpenSnapshot` callback is triggered. This function directly calls the
`onViewModeChange('single')` prop instead of the new `handleViewModeChange` wrapper. As
a result, the `trackAnalytics('preprod.snapshots.details.view_mode_changed', ...)`
event, which is located inside `handleViewModeChange`, is not fired. This leads to a
loss of analytics data for a primary user interaction, contradicting the PR's stated
goal of centralizing analytics wiring within the `SnapshotMainContent` component.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[github-actions]

📊 Type Coverage Diff

Metric	Before	After	Delta
Coverage	93.59%	93.59%	±0%
Typed	133,253	133,254	🟢 +1
Untyped	9,128	9,128	±0

🔍 1 new type safety issue introduced

Type assertions (as) (1 new)

File	Line	Detail
static/app/views/preprod/snapshots/main/snapshotsToolbar.tsx	163	as Node — e.target as Node

This is informational only and does not block the PR.