fix(desktop): quote Windows Start at Login executable path

[Yuxin-Qiao]

Summary

• Windows desktop Start at Login currently writes an unquoted executable path, so an installation under Program Files produces an ambiguous startup command. Write "<executable>" --autostart through a Windows-only registration helper for first-run setup, manual enables, and migration.
• Revisit the old migration marker once on Windows, rewriting only entries already enabled. Preserve tray/Task Manager opt-outs, the plugin registration name, and explicit-enable behavior. macOS and Linux continue using the plugin.
• Update the desktop guide and architecture contract. No interface changes.

Closes #6473

Verification

• Desktop Bun tests: 296 passed, 2 Windows-only PowerShell cases skipped on macOS.
• Rust command formatter: 3 tests passed (Program Files, Unicode/no-space paths, invalid paths).
• Windows-target cargo check of the production registration helper against winreg 0.55.0 passed in an isolated compile harness with a stub AppHandle; this does not validate the complete Windows application.
• Native macOS cargo check --locked passed with TAURI_CONFIG excluding missing generated bundle resources. No packaged build or Windows login/registry integration test was performed.
• TypeScript typecheck, privacy scan, structure checks, Rust formatting, and documentation build (561 pages) passed.
• Full repository tests were not run for this desktop-scoped draft. Keep draft pending full validation, Windows verification, and explicit security review of startup registration/dependency handling.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed.
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

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✅ Deterministic PR hygiene checks passed.

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⏳ DRAFT

• review readiness checklist open (0/4 boxes ticked).

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• Tick all four boxes in the PR description once you're done (currently 0/4).

Review readiness checklist

• ⬜ Required local validation passed; commands, results, and any full-suite exception are documented.
• ⬜ I pushed my PR to a recent dev commit (at most 10 behind; a maintainer may still ask for the exact tip before merge).
• ⬜ I resolved all correct Codex and CodeRabbit findings.
• ⬜ My PR is ready for review.

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This PR stays in draft until every box above is ticked.

Hygiene

✅ Deterministic PR hygiene checks passed.

[lidge-jun]

Superseded by the integration in #6487, with reviewed follow-up fixes in #6490 and Windows validation repairs in #6494/#6495, all merged into dev.

The Windows autostart executable quoting implementation was carried. Real Start at Login acceptance remains tracked separately in #6473; closing this replaced code PR does not close that issue.

Original carry commit: b0b884b2c940ed80de3e47256bb2f878882679c1. Attribution to @Yuxin-Qiao is preserved in the integration history and merge trailers. The final integrated candidate passed the complete cross-platform CI run.

Closing this PR as superseded, not claiming that its original head was merged. Thank you for the contribution.