Avoid eager work in host file previews

[ymichael]

What was wrong

PR #2005 added an absolute-host preview query that treated heavy preview work as eager and lightweight: it read the complete file and built a base64 fallback before attempting the preview lease, kept the retained panel query enabled after the panel closed, and used React Query's default five-minute payload retention. A successful media or HTML lease could therefore duplicate host I/O, while a hidden panel could start or continue a large read and keep its payload observed indefinitely. This addresses the connected findings in the eager payload review, the hidden-panel review, and the retention review.

What changed

• Pass the primitive plugin-panel isOpen state through the existing lazy host-preview boundary and use it to gate useHostFilePreview.
• Move a disabled retained observer off the active host/path key so closing the panel aborts the existing SDK request and starts cache GC without unmounting or changing the lazy-loading boundary.
• Attempt the existing sdk.files.createPreview lease first. Successful image/video leases now return a lightweight URL-backed preview without reading or retaining file bytes. Text and HTML still read the source bytes they render, but no longer build a base64 fallback after a lease succeeds; lease failures build a data URL only for preview kinds that need one.
• Apply HEAVY_PAYLOAD_QUERY_POLICY, giving inactive host-preview payloads the shared one-minute retention period.
• Cover hidden/open/reopen gating, cancellation and GC, lease-first media and fallback behavior, HTML source preservation, and the ambiguous .ts source-preview case.

This is direct app query/component plumbing over the existing SDK APIs. It adds no server or daemon contract fields, so HOST_DAEMON_PROTOCOL_VERSION is unchanged. It also needs no CLI, guide, or public SDK documentation change.

How you verified

• Before the implementation, pnpm exec turbo run test --filter=@bb/app -- src/hooks/queries/host-file-preview-query.test.tsx src/components/secondary-panel/ThreadSecondaryPanelTabContent.panelGate.test.tsx src/components/plugin/PluginPanelRightPanelHost.test.tsx reproduced the findings: 5 failed and 13 passed. The failures showed the hidden initial read, missing isOpen plumbing, media byte read, base64-before-lease HTML work, and read-before-failed-lease ordering.
• After rebasing onto origin/main at 7f3d2ac66, the same focused command passed 3 files / 20 tests.
• pnpm exec turbo run test --filter=@bb/app --force passed 412 files / 3,162 tests, with 3 skipped.
• pnpm exec turbo run typecheck build --filter=@bb/app passed.
• pnpm exec turbo run lint --filter=@bb/app passed with 0 errors and 156 existing warnings outside the changed lines.

Post-merge follow-up to #2005; no matching issue.

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