feat(desktop): Config tab to pin the oab-mcp target + reload core (UI)

[brettchien]

The UI half of the oab-mcp target feature — stacked on #60 (feat/mcp-target-hermetic, which adds the backend + mcp_target_get/mcp_target_set commands). Base is #60's branch; will retarget to main once #60 merges.

Finishes #10's intent (Config tab) on #60's provider-tagged design.

What

• New Config tab (console/index.html + config.ts): cluster / AWS profile / region fields. Reads the persisted target via mcp_target_get; on save calls mcp_target_set (persist + reload the core onto the new hermetic env), then refreshes the roster. Browser build disables the form (no core).
• Maps to feat(desktop): provider-tagged, hermetic oab-mcp target (backend) #60's tagged shape { provider: "ecs", cluster, profile?, region? } — a future provider adds its own section.
• main.ts: one initConfigTab({ onSaved: () => void tick() }) call at boot; tab-switching is already generic.
• styles.css: config form styling using existing theme tokens.

Verification

• tsc --noEmit && vite build clean locally (46 modules, no type errors) — the TS/HTML/CSS is verified on this box (unlike src-tauri, which needs the macOS runner).
• The backend commands it calls are feat(desktop): provider-tagged, hermetic oab-mcp target (backend) #60's, gated by feat(desktop): provider-tagged, hermetic oab-mcp target (backend) #60's bundle-macos.

Sequence

1. Merge feat(desktop): provider-tagged, hermetic oab-mcp target (backend) #60 (backend) once its bundle-macos is green.
2. Rebase this onto main, merge.
3. Close feat(desktop): Config tab to pin oab-mcp target + reload core #10 (superseded by feat(desktop): provider-tagged, hermetic oab-mcp target (backend) #60 + this).

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