Add per-pane new thread draft slots

[brsbl]

What was wrong

New-thread compose state used one browser-wide draft identity. Opening a second new-thread pane reused that singleton, so two panes could not preserve independent unsent text or restore the focused draft reliably.

What changed

• adds client-local draft slots with stable identities and submit destinations
• keys each app-owned new-thread pane to its own slot while leaving plugin-rendered composers outside Drafts state
• migrates legacy singleton draft and split-layout persistence deterministically
• preserves slot identity through split navigation, focus, close, remount, and reload

No server state, wire contract, or daemon protocol changes are introduced by this layer.

Visual evidence

Exact comparison: 977d04e4236b68433bd068139909a3e129ed7b38 → a2f8eea7dedf54396c68500d826f2ab3772a0cc3.

Before — exact base 977d04e4236b68433bd068139909a3e129ed7b38

Modified New thread still resolves to the singleton composer.

After — exact head a2f8eea7dedf54396c68500d826f2ab3772a0cc3

The same action creates a second independent new-thread pane.

After — independent text survives reload

How you verified

• Google Chrome for Testing 151.0.7922.71, exact branch web apps from scripts/bb-dev-app, light theme, /, 1440×900.
• On the base, Meta-clicking New thread left one composer. On the head, the same interaction produced two panes.
• Entered “Left independent draft” and “Right independent draft” in separate panes, hard reloaded, and observed both strings restored with the right pane focused. No page or console errors were recorded.

Fixes: N/A — Updated Sidebar draft-slot layer.

BB-Thread-ID: thr_2e2gbjx943

AGENT GENERATED