Unify instance eject with optional weight deletion

[alytaphoenix]

Summary

• Renames the instance card's DELETE button to EJECT and unifies two previously-separate actions into one flow:
  1. Stop the running instance (existing DELETE /instance/{id}, unchanged).
  2. On success, optionally also delete that instance's downloaded weights from disk (existing DELETE /download/{node_id}/{model_id}, unchanged), via a second confirmation — no more navigating to the Downloads page separately.
• No backend changes; both endpoints already existed and are reused as-is.
• Weights are deleted only on the nodes this instance ran on (shardAssignments.nodeToRunner). If the same model's weights happen to exist on other nodes from a prior placement, those are untouched — this eject action only clears the weights for the instance being ejected.

Known limitation

DELETE /instance/{id} enqueues a DeleteInstance command and returns immediately — actual teardown happens asynchronously in the master's reconciliation loop. The weight-deletion confirmation fires right after that 200 response, not after teardown completes. Ejecting an instance that's still PREPARING/loading and choosing to also delete weights means the delete-from-disk can race the in-flight load on that node. The failure mode is bounded (the file unlink itself succeeds; a mid-load error surfaces as a log/cosmetic error on that node; the model is just re-downloadable) but it's worth knowing about if you eject early in a model's load.

Test plan

• npx svelte-check — no new errors (same baseline as main: 15 errors / 6 warnings)
• npm run build — succeeds
• Verified in a real headless browser (Playwright) with mocked /state, /instance/{id}, and /download/{node}/{model} endpoints (no real model instance was loaded — this sandbox has no mlx installed, so a live inference instance isn't reachable here): confirms the EJECT button renders, the two sequential confirm dialogs fire with the right copy, DELETE /instance/{id} fires first, and DELETE /download/{nodeId}/{modelId} fires second with the model ID correctly URL-encoded.
• This is a dashboard-only change (no Python touched), so basedpyright/ruff/pytest are not applicable here.
• nix fmt / nix flake check were not run (nix unavailable in this sandbox) — substituted prettier-plugin-svelte directly, unconfirmed against the real treefmt config.
• Note: this repo has no component-level tests for the dashboard (traces/downloads pages ship without them either), so no test file was added for this change, consistent with existing precedent.

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https://claude.ai/code/session_011rjSfwDBTkmySmfU6NgHKF