Add the portable settings document for settings sync

[enaboapps]

Closes #985
Part of #988.

What

Adds src-tauri/src/portable_settings.rs, the versioned document (schemaVersion: 1) that the sync engine (#986) will store in desktop_preferences.payload.

Synced:

• App settings: pointer speed, mouse and key repeat, dwell, cursor overlay
• Custom (non-built-in) switch profiles
• Switch settings, including physical switch keys, hold interval and hold actions
• All point-scan config and scan preferences, plus the keyboard layout
• The 8 remote switch slots

Kept local: desktopId, paired devices and pairing tokens, telemetry install id and consent, setup flags, startWithSystem, the remote config revision counter, the current scan mode, and the legacy point-scan key fields (superseded by switch bindings).

Deviation from the plan: shareDiagnostics is also kept local. It is the telemetry consent, and consent should be given on each install rather than arrive from another machine.

Applying incoming documents

• parse rejects documents with a newer schemaVersion (ParseError::Newer, with an "update Switchify PC" message) as well as malformed ones. Unknown fields are rejected, so additions require a schema bump.

• plan validates every section before anything changes. App settings use normalized(); switches and remote slots use validate(); point scan uses Config::validate(); profiles use the same validate_profile rules plus the uniqueness and 32-profile limits.

• plan also merges in local-only values and returns only the sections that differ, so unchanged sections are never re-applied. (A point-scan apply resets the scanner.)

• apply runs on the main thread and goes through the existing save paths:

  • switches: switch_runtime::Controller::save (pausing point scan first), then remote_scan::apply
  • remote slots: remote_scan::save
  • point scan: point_scan_runtime::configure
  • keyboard layout: update_point_setting
  • app settings: apply_app_settings, newly extracted from save_settings so startup, consent, dwell, repeat and overlay side effects stay identical
  • profiles: persisted through AppModel::persist

  It refuses to run during switch practice, like the settings commands.

• Profile versions: a profile whose bindings, name or provider changed gets a version above both copies, so connected phones pick it up. An unchanged profile keeps its local version.

Nothing calls the module yet; #986 wires it into the sync engine. It carries #![allow(dead_code)] until then.

Validation

• npm run prediction-model, npm run lint, npm test (254 passed) and npm run build
• cargo fmt --check and cargo clippy --all-targets -D warnings: clean
• cargo test: 660 passed, 3 ignored, including 21 new portable_settings tests covering:
  • device-only values never entering the document
  • switch keys being included
  • round-trip with no planned changes
  • rejection of newer, malformed and unknown-field documents
  • local-only settings, scan mode, legacy keys and remote revision being kept
  • keyboard layout being split out
  • every invalid section being rejected before applying
  • profile validation, the limits, version bumping, unchanged versions and removal

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