fix: keep install state across Home navigation - #49

Problem

While an instance is installing, leaving Home for Settings and returning shows:

Error: installation already in progress for <id>

even though the original install keeps running and finishes. Home is remounted with the same activeInstanceId, so loadAndInstall calls InstallInstance again. The backend correctly refuses the second pipeline, but the UI renders that refusal as a failure.

Changes

• Move install progress/error state from Home.svelte into a global installStore so it survives navigation.
• Guard handleInstall and the auto-install path so a remount or double click reuses the running pipeline.
• Keep real failures visible; only auto-start when the install state is idle.
• Make InstallInstance idempotent: if the per-instance claim is already held, log and return nil instead of returning an error.

Verification

• npm run check — 0 errors
• npm run build
• go test ./...
• git diff --check

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