fix(ui-desktop): unify path resolution around GOOSE_PATH_ROOT

[bavadim]

Summary

This PR keeps path resolution consistent in Desktop by using GOOSE_PATH_ROOT as the path-root contract, with a minimal surface area.

What changed

• Desktop main.ts: expose/pass GOOSE_PATH_ROOT for Desktop runtime and renderer config.
• Desktop MCP shim scripts (jbang, uvx, node-setup-common.sh): resolve mcp-hermit under GOOSE_PATH_ROOT/config (or platform default when unset).
• Desktop recipe storage picker: use GOOSE_PATH_ROOT/config/recipes for global recipe browsing in isolated runs.

Why

In branded/isolated Desktop runs, backend paths and UI/shim paths can diverge (GOOSE_PATH_ROOT/... vs ~/.config/goose/...).
That mismatch can cause extension bootstrap and recipe UX inconsistencies.

Related issues

• Related to Failed to perform one time cleanup of mcp-hermit #5944 (mcp-hermit cleanup/path robustness in desktop flows)
• Related to File picker for scheduling recipes should default to recipe dir #3431 (desktop recipe picker default path)
• Related to any MCP server extensions I add to the goose config fail to start when running goose UI #5048 and Bug: Unable to add extension on linux #2351 (desktop MCP setup inconsistencies vs CLI on Linux)

Validation

• cd ui/desktop && npm run lint:check
• cd ui/desktop && npm run test:run

Both passed.

AI usage

AI assistance was used to draft and apply code edits; validation was performed by running the checks above.