fix(web): stop desktop startup from crashing

[tusharbhardwaj-bk]

The staging desktop renderer crashed during module initialization because the thread route constructed an empty EnvironmentId placeholder. Since EnvironmentId enforces a non-empty string, React never mounted and the static splash screen remained visible.

This moves optional connection lookup into a fork-owned null-safe hook and keeps the upstream route seam to one import and one hook call. It also adds focused reachability coverage for missing, disconnected, synchronizing, and connected states.

Verification:

• vp test run apps/web/src/fork/threadHostReachability.test.ts apps/web/src/threadSync.test.ts
• vp run --filter @t3tools/web typecheck
• targeted formatting and lint
• fork marker check
• isolated Electron dev launch reached the project picker past the splash

Generated by GPT-5.6-sol via Codex in T3 Code.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	13.5 KiB	—	15.1 KiB	✅
Codex	Thread snapshot wire	—	7.2 KiB	—	7.3 KiB	✅
Codex	Live turn WebSocket wire	—	6.3 KiB	—	7.8 KiB	✅
Codex	Live turn WebSocket decoded	—	55.6 KiB	—	66.4 KiB	✅
Codex	Live turn messages	—	9	—	21	✅
Claude	Total thread wire	—	13.5 KiB	—	15.1 KiB	✅
Claude	Thread snapshot wire	—	7.2 KiB	—	7.3 KiB	✅
Claude	Live turn WebSocket wire	—	6.3 KiB	—	7.8 KiB	✅
Claude	Live turn WebSocket decoded	—	56.4 KiB	—	66.4 KiB	✅
Claude	Live turn messages	—	9	—	21	✅

Baseline: unavailable · PR result: 54aeff1 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 110.5 KiB
• Claude decoded thread snapshot: 111.1 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.