fix(desktop): prevent proxy password mask corruption

[Sun-GLiang]

Summary

• keep saved proxy credentials out of persisted settings and Renderer input values
• add an explicit keep/replace/delete credential contract with a shared Runtime Host operation lane
• keep password editing local until blur or Enter, make Eye reveal the current draft, and remove Copy only from the proxy password field
• preserve schema-v1 config import/export compatibility with preflight validation

Fixes #3696

Dependency

• Built on fix/proxy-auth-test-consistency, the implementation branch for bug(desktop): proxy test drops enabled authentication when credentials are empty #3682.
• Until bug(desktop): proxy test drops enabled authentication when credentials are empty #3682 lands, the compare view against main also contains dependency commit ec3b38b3c.
• Rebase this branch onto the updated main after bug(desktop): proxy test drops enabled authentication when credentials are empty #3682 lands so the PR contains only b94e9ab0d.

Verification

• npm test — all workspace tests passed; Desktop 1393/1393
• focused proxy password tests — 36/36
• offline authenticated proxy E2E — 1/1
• npm run lint
• npm run format:check
• npm run build
• npm run typecheck
• npx knip --workspace apps/desktop
• npx knip --workspace packages/ui

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex implemented the fix, tests, verification, and review-driven corrections.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No