Add sidebar Split lifecycle

[brsbl]

What was wrong

The primary New thread control could replace the current pane but could not deliberately create the spec's two-pane drafting workspace. Draft reopening and submission also lacked pane-local split lifecycle behavior.

What changed

• adds a trailing Split action to the New thread row on hover/focus
• replaces the workspace with exactly two fresh blank composers and focuses the left pane
• removes the action in compact viewports
• keeps new-thread submission pane-local and supports reopening an existing draft in the split
• uses internal app behavior only; this layer does not add a public command ID or bump the plugin SDK

Visual evidence

Exact comparison: 4a78545610576c1839ca271cb9440be657e93874 → fd1c3ea60b4ffeafefeab4c5bff8b893d8364531.

Before — exact base 4a78545610576c1839ca271cb9440be657e93874

The hovered New thread row has no trailing split action.

After — exact head fd1c3ea60b4ffeafefeab4c5bff8b893d8364531

The same route, fixture, lifecycle filters, draft, viewport, and hover state reveal Split.

After — split result

After — compact viewport

How you verified

• Google Chrome for Testing 151.0.7922.71, exact branch web apps from scripts/bb-dev-app, light theme, /, 1440×900 plus the scoped 700×900 compact check.
• Hovered New thread, activated Split, and observed exactly two blank composers with the left editor focused.
• At 700 px, inspected the same row and confirmed the Split control was absent. No page or console errors were recorded.

Fixes: N/A — Updated Sidebar split layer.

BB-Thread-ID: thr_2e2gbjx943

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