docs(rfc): propose Tauri desktop control center

[knqiufan]

Which issue or RFC does this PR close?

Refs #1428. This PR proposes the desktop design; implementation and delivery remain tracked by #1428.

Rationale for this change

Users currently need several installation, configuration, and diagnostic surfaces to manage PowerContext. The desktop should provide one native control center while preserving the existing Python Server, public API, and independent service lifecycle.

The proposal chooses Tauri 2 with shared Web presentation and a narrow Rust host. It compares Electron fairly and makes authorization, durable Handoff delivery, installation, and distribution dependencies explicit.

What changes are included in this PR?

• Add synchronized English and Chinese RFCs covering product scope, component ownership, public API reuse, connections, credentials, resource authorization, Handoff, notifications, Sources, and integrations.
• Define installation, updates, failure recovery, data-preserving uninstall, diagnostics, localization, and accessibility requirements.
• Propose Windows 11 x64 with SQLite as the first qualified platform, a phased delivery plan, and 18 observable acceptance criteria.
• Add both RFCs to the documentation navigation.

Are there any user-facing changes?

Documentation-only design proposal. This PR does not change runtime behavior, public APIs, dependencies, or persisted formats. The RFC distinguishes a personal preview from the full collaboration and delivery requirements for #1428.

How was this change tested?

• uv lock --check.
• uv run --locked prek run --all-files, including the configured ty check hook.
• Strict python -m zensical build --clean -s using the project virtual environment and an isolated tracked-source snapshot with the proposed documentation. Verified the generated API landing page and byte-for-byte equality of the published OpenAPI contract, matching the make docs-test artifact checks.
• Checked local Markdown links, matching English/Chinese section and acceptance identifiers, TOML parsing, and git diff --check.

AI usage statement

OpenAI Codex assisted with repository and issue research, drafting and translating the RFC, validation, and PR preparation.