[MCP marketplace] Add read-only Browse catalog in Settings → MCP

[GoDiao]

Parent: #12
Depends on: #44

Goal

Let users discover curated MCP servers from the existing Settings → MCP page without changing their configuration.

Scope

• Add an Installed/Browse navigation model inside McpSettings.
• Load and validate the catalog through a path that works in both development and packaged Tauri builds.
• Show searchable/filterable server cards with description, tags, homepage, command summary, runtime requirements, and required environment variables.
• Indicate when an entry name is already installed.
• Add loading, empty, and invalid-catalog states.
• Cover the catalog model and main Browse interactions with tests.

Done when

• The catalog is available in development and in the packaged application layout.
• Browse does not mutate MCP configuration.
• Invalid catalog data fails closed and produces a useful UI state.
• Existing installed-server management remains unchanged.

Out of scope

• Install/configuration writes.
• Secret collection.
• HTTP/SSE catalog entries.