fix(web): offer recovery from missing pages

[shivamhwp]

An unknown application URL showed only the router's bare "Not Found" text. The page now explains the problem and offers a Go home link, replacing the invalid history entry so users can return to a project or thread.

Refs #5591. This fixes the visible dead end reproduced at an invalid route; the report's original repository-removal trigger remains unverified. Stacked on #11298. The diff changes only the root route's missing-page view.

The web typecheck passes, and both scoped lint warnings match baseline. In the actual Linux Chromium client, the invalid route shows the new recovery view and clicking Go home reaches a usable draft composer. Before/after images show the same invalid URL. No repository was deleted, and native macOS/Windows, mobile and hosted-static onboarding were not exercised.

Before:

After:

Implemented and tested with GPT-6 through Codex in T3 Code.

[macroscopeapp]

Approvability

Verdict: Approved at d7f9d49

Macroscope's review found this PR approvable — This single-file fix replaces the bare missing-page response with a small explanatory view and an existing home navigation action. Its runtime impact is confined to invalid routes, with no changes to valid paths, defaults, schemas, infrastructure, or static-analysis configuration.

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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	—	13.6 KiB	—	15.1 KiB	✅
Codex	Thread snapshot wire	—	7.0 KiB	—	7.3 KiB	✅
Codex	Live turn WebSocket wire	—	6.5 KiB	—	7.8 KiB	✅
Codex	Live turn WebSocket decoded	—	57.1 KiB	—	66.4 KiB	✅
Codex	Live turn messages	—	10	—	21	✅
Claude	Total thread wire	—	13.6 KiB	—	15.1 KiB	✅
Claude	Thread snapshot wire	—	7.0 KiB	—	7.3 KiB	✅
Claude	Live turn WebSocket wire	—	6.5 KiB	—	7.8 KiB	✅
Claude	Live turn WebSocket decoded	—	57.8 KiB	—	66.4 KiB	✅
Claude	Live turn messages	—	9	—	21	✅

Baseline: unavailable · PR result: d7f9d49 · 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: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

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