fix(ui): preserve native open actions and password prompts

[skylartaylor]

What this changes

Choosing Export Text from Home opened the picked PDF on the native worker but forgot the selected tool. The request now retains its command through preparation and password retries, and runs it after the requested document commits. Successful text/image conversion uses the same continuation; cancellation and failed opening leave existing documents alone.

A startup download could also create its password prompt while local authentication was pending. Completing the local open cleared that unrelated prompt. Startup intake now waits for pending native preparation while retaining the existing modal and remote-link permission guards.

The native UI tests now wait for document/password completion and interactive controls, retaining their content, active-document, dirty-state, destination and recovery checks. Refused linked-PDF targets now await their specific validation error and verify the source document stays intact. The byte comparison retains the exact generated fixture written to disk, including its timestamp. The wheel test follows the configured platform speed. The invalid-filename fixture reports macOS EILSEQ capability limits explicitly and runs a valid-Unicode batch companion; supporting filesystems still exercise the original invalid-name case, and other I/O errors fail.

How you verified it

• Eight controlled native worker tests passed. Reverting the production fixes on the reviewed main reproduced five intended failures; restoring them passed all eight again. Cases include another document opening during preparation, conversion, wrong/correct passwords, picker/password cancellation, failed input and background startup prompts.
• cargo xtask ci: all eight gates passed; 3,018 native tests passed, 14 existing ignored, zero failures. The 31 library crates compiled for Wasm. Strict native Clippy passed; inherited target-specific Wasm warnings remain.
• Formatting, strict all-target Clippy, native tests, assets, layers, dependency audit, parity and Wasm compiler checks are covered by the local gate receipt. Wasm compilation is separate from browser execution.
• Synthetic fixtures only. Printing transports and destinations were rejected during testing.

Review and scope

The broader opening work in #1634 overlaps these continuations; this is a focused repair of the two regressions on current main following #1493. Product changes are confined to the UI opening path, with permanent regressions and scoped test/documentation updates. Parity statuses and estimates are unchanged.

No new dependencies, assets, fonts, unsafe code, product panic paths or network access. The existing opt-in control/MCP behavior and root confinement are retained.

[skylartaylor]

Windows CI caught one linked-PDF fixture expecting an error after three frames. The held-worker trace confirms validation is still pending then. The tests now await the intended refusal and compare the exact bytes written to disk, retaining the original document checks. All local gates pass; I’m waiting for complete CI on the new head before merging.