feat(invoke): add image-board selector for reference uploads

[lstein]

Summary

• Reveals the InvokeAI username/password fields and a new "Upload to Image Board" dropdown in the settings dialog only after the configured URL probes as a working InvokeAI backend.
• Reference images uploaded via Use as Ref Image are placed in the selected board; if the board can't be fetched at settings-load time, or the configured board rejects the upload at runtime, the image lands in Uncategorized and the response carries a warning.
• Independent of feat(invoke): show "Use as Ref Image" button for any image when InvokeAI is configured #221 (no overlapping files) — branched off master so the two PRs can land in either order.

Backend

• Config.invokeai_board_id field; to_dict / load_config / get_invokeai_settings / set_invokeai_settings round-trip it.
• GET /invokeai/status probes the unauthenticated /api/v1/app/version. Returns {reachable, version, detail}. Used by the settings UI to decide whether to reveal the auth section. Treats a 200 without a version field as not-InvokeAI.
• GET /invokeai/boards fetches /api/v1/boards/?all=true through the existing auth-fallback helper (renamed _post_with_auth_fallback → _request_with_auth_fallback since it's now used for GET as well). Returns a flat [{board_id, board_name}] list; handles paginated responses too. Any failure surfaces as 502.
• GET /invokeai/config adds board_id to the response. POST /invokeai/config round-trips it with the same preserve-on-null / clear-on-empty-string semantics as the password field.
• _upload_image_to_invokeai accepts board_id and retries without it on a 4xx, returning a warning string the caller surfaces. /use_ref_image includes warning in the response on fallback.

Frontend

• settings.html: wraps username/password rows + new <select id="invokeaiBoardSelect"> in #invokeaiAuthSection (hidden by default).
• settings.js: load → /invokeai/status → conditional reveal → /invokeai/boards. URL/user/pass autosave (debounced) and re-probe; board selection autosaves on change. On boards fetch failure the dropdown is disabled and shows only "Uncategorized".

Test plan

• ruff check photomap tests clean.
• pytest tests/backend — 119 passed (+12 new: status reachable / unreachable / non-InvokeAI / paginated boards / board_id round-trip / preserve-on-null / clear-on-empty / boards 502 / upload uses board_id / fallback to Uncategorized + warning).
• npm test — 239 passed (+6 new in invokeai-settings.test.js covering reveal/hide, board population, and fallback).
• npm run lint and npm run format:check clean.
• Manual: enter a valid InvokeAI URL → username/password/board fields appear, dropdown populates.
• Manual: enter junk URL → fields stay hidden.
• Manual: pick a board, click Use as Ref Image → image lands in that board in InvokeAI.
• Manual: delete the board in InvokeAI without updating settings → next Use as Ref Image lands in Uncategorized and surfaces the warning.

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