feat: pre-release update channel, safe mode crash protection, and pipeline backup

[Horuse]

What does this PR do?

Prepares Splitwave for v1.2.0 testing and public release with rollback safety, crash protection, and an opt-in pre-release update channel:

1. Pre-release Update Channel (Settings -> Updates):

   • Adds "Receive pre-release updates (Beta / RC)" toggle in Settings.
   • When enabled, the in-app updater resolves the latest pre-release endpoint (-rc, -beta, -alpha) from the GitHub Releases API instead of only stable releases.
   • Displays a distinct PRE-RELEASE badge on the update dialog when targeting pre-release builds.
   • Automatically marks releases as prerelease in GitHub Actions if the git tag contains -rc, -beta, or -alpha.

2. Automatic Pre-1.2.0 Pipeline Backup & Downgrade Safety:

   • Rust backend automatically snapshots pipelines.json to pipelines.backup-v1.json on app startup if the backup does not already exist.
   • Preserves backward compatibility for fileRecording nodes by syncing allowOverwrite = (mode === 'overwrite') on save and during migrations, allowing seamless rollback to v1.1.0 without losing overwrite intent.

3. Safe Mode Crash Loop Protection:

   • Sequences error handler initialization (take_crash_reports) before audio pipeline auto-activation on startup.
   • If the previous run ended in a crash or audio boot failure, audioStore.autoActivateOnLaunch() skips auto-starting the pipeline to prevent boot crash loops.
   • Displays an amber Safe Mode alert banner situated directly beneath the header
   • Error modal displays a prominent SAFE MODE indicator explaining that auto-start was disabled.

4. Dev Tools:

   • Added triggers to test "Safe Mode (simulate)" and "Pre-release update available" alongside existing debug options.

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Why is this the right approach?

• Pre-release channel: Allows testing v1.2.0-rc builds with early testers directly through the updater without forcing manual GitHub asset downloads or affecting stable users.
• Automatic backup: Backing up pipelines.json in Rust at startup before any frontend JavaScript or migration code runs guarantees preset preservation even if an unexpected edge case fails during initial load.
• Safe Mode: Startup crash loops (e.g., from unplugged hardware or crashing audio drivers) are prevented without clearing the user's saved pipeline configuration.

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Checklist

• Diff is limited to the change — no unrelated edits
• bun run check passes (0 errors)
• cargo check --manifest-path src-tauri/Cargo.toml passes
• Generated TS types are committed with the Rust change (if any)
• No RT audio path violations (no allocations, locks, or syscalls in audio callbacks or DSP worker)

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Platform coverage

• Developed on: macOS
• Tested on: macOS (aarch64)
• What I did to test:
  • Verified pre-release updater endpoint resolution and PRE-RELEASE modal badge.
  • Verified automatic backup creation of pipelines.backup-v1.json in app data directory.
  • Verified downgrade compatibility with fileRecording node mode/overwrite sync.
  • Tested Safe Mode recovery flow on native process crash / panic and simulated Safe Mode trigger.
  • Verified layout responsiveness across all routes (+page, pipelines/[id], settings, virtual-devices, wiki).