Refine desktop settings center / 优化桌面设置中心

[SivanCola]

Summary

• Rework the desktop settings center into a unified modal with embedded MCP, Skills, and Memory pages.
• Refine model provider access: official/custom provider flows, model discovery, effort mapping, and provider key handling.
• Redesign Memory settings into Memory entries and Instruction files, with inline add-memory and scoped instruction previews.

Verification

• npm run build (desktop/frontend)
• go test ./internal/config ./internal/provider/openai
• go test . -run 'Test(RemoveEnvFileDeletesKeyAndUnsetsProcessEnv|UpsertEnvFile|PromoteProviderKeys|Settings|Provider|TabProfile)' (desktop)\n- Browser checks for settings control alignment, custom provider flow, and Memory tab structure\n

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve provider-only fields when refreshing models

When a user refreshes models for an existing provider, this sends the frontend ProviderView back through SaveProvider; that view does not carry provider-only config such as no_proxy or price, while UpsertProvider replaces the whole entry. For the new official Mimo templates that set NoProxy: true (and for any priced existing provider), clicking refresh silently clears those fields, so subsequent requests can stop bypassing the proxy for endpoints that require it and cost reporting is lost. Please update only the model list/default here or round-trip the hidden provider fields.

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[SivanCola]

Superseded by the newly split main-v2 based PRs, especially #3365 for settings navigation copy and #3366/#3368 for the related desktop UI work. This PR is broader than the remaining review scope and overlaps with the focused PR stack, so it is being closed. Any remaining provider/model configuration pieces can be resubmitted as a smaller dedicated PR.