fix(web): keep sidebar thread order across restarts and make new rows draggable

[JTBroad]

Fixes two separate defects in the v2 sidebar's drag-to-reorder.

1. Persisted order was wiped on every restart

The prune effect only gated on "is an environment connected". On a cold start the environment descriptor arrives before the thread snapshot does, leaving a window where every environment looks connected with zero live threads. pruneThreadOrder then dropped every persisted key and the list fell back to the default created-date sort.

Gated the effect on useAllEnvironmentShellsBootstrapped() so an absence of threads is only trusted once the snapshots have actually landed.

2. New threads snapped back when dragged

reorderThreads can only splice keys already present in threadOrder, and unranked threads are absent from it by design — orderThreadsByPreferredKeys deliberately floats never-dragged threads to the top, outside the order array. So removed.length === 0 and the reducer returned state unchanged.

The existing seed-from-rendered-list fallback only applied when threadOrder was entirely empty, so it never helped once the user had dragged anything.

Now the drag baseline folds unranked rendered keys in at their displayed position, so any visible row is draggable. This also covers dragging a thread that is actively working, since that is usually a recently created one.

New threads still appear at the top by default — the unknown-first ordering is untouched; the first drag simply pins them into the order. No change was needed at the thread-creation site.

Reviewer notes

• Single file changed: apps/web/src/components/SidebarV2.tsx.
• tsc --noEmit clean; existing uiStateStore.test.ts (15 tests) passes.
• No new tests. The drag baseline is easy to cover in uiStateStore.test.ts; the prune race needs a component-level test. Happy to add both if wanted.
• Not verified against the running app. Both fixes are reasoned from the code, not observed end-to-end — the restart fix especially deserves a manual close/reopen before merging.

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