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

[brettchien]

Why

The deploy_list AccessDeniedException Brett hit wasn't a missing IAM policy — it was credential/region drift. The desktop spawned the oab-mcp sidecar with only OAB_CLUSTER, so it inherited the host's ambient AWS default chain and pointed at the wrong account/region (916371022086 / ap-southeast-1), where the oab cluster doesn't exist. The cluster actually lives in 504190915686 / ap-east-2.

Fix: make each oab-mcp's target explicit and persisted, and let the user change it from the UI without restarting the app.

What

• src-tauri/src/config.rs (new) — McpConfig { cluster, profile?, region? }, persisted to the app config dir as mcp-config.json. First-run defaults seed from the process env so behaviour is unchanged until the user saves.
• src-tauri/src/mcp.rs — spawn() takes the config and builds a curated child env: inherit the parent env minus every AWS_*, then re-inject the configured AWS_PROFILE / AWS_REGION (+AWS_DEFAULT_REGION). This is deliberate: a stale ambient AWS_ACCESS_KEY_ID would otherwise outrank AWS_PROFILE in the SDK chain and re-introduce the drift. The child is now held in an Option and killable via shutdown() for reload.
• src-tauri/src/lib.rs — get_config / set_config commands. set_config persists, then kills + respawns the core with the new env. deploy_list now defaults to the configured cluster. Config is loaded into managed state at startup.
• console/ — third Config tab (index.html, config.ts, styles.css): a profile / region / cluster form. Save calls set_config, then refreshes the roster against the new target. Browser build disables the form.

v1 is a single target, shaped to grow into a multi-fleet registry (provider-tagged connections) — the direction discussed in the thread.

Verification

• ✅ console: npm run typecheck, npm test (8/8), npm run build — all green.
• ⚠️ Rust: not compiled in my env — the ECS container lacks the GTK/WebKit system libs Tauri needs (glib-sys build fails on missing glib-2.0.pc). I verified every external API against the pinned crate sources instead:
  • Command::env_clear() / envs<I,K,V> / sidecar() -> Command — tauri-plugin-shell 2.3.5
  • CommandChild::kill(self) — same
  • PathResolver::app_config_dir() -> Result<PathBuf>, Manager::path() — tauri 2.11.5
  • command-arg default is camelCase (new_config ↔ newConfig) — tauri-macros 2.6.3
  • Please run cargo build/cargo check in src-tauri/ on the host to confirm the compile.

How to test on the host

1. Build & run the desktop app (cargo tauri dev or the bundled build).
2. Open the Config tab → set profile brettchien, region ap-east-2, cluster oab → Save & reload core.
3. Activity pane should log config saved — reloading core… then a fresh spawn; roster should list the oab services (oab-prod-mira, oab-prod-orca) instead of the AccessDenied error.
4. Config persists across restarts (mcp-config.json in the app config dir).

Base is feat/desktop-mcp-sidecar (not main) because this builds directly on the sidecar work that isn't merged yet.

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