Remember the scanning panel position between sessions

[enaboapps]

Closes #1039

What changed

• New panel_position module stores the chosen scanning panel dock locally in panel-position.json ({"column":C,"row":R}) in the app config folder, next to point-scan.json and switch-settings.json. Writes replace the file atomically through the existing storage::replace_file helper.
• A missing, unreadable, malformed or out-of-range file falls back to the default bottom-middle dock.
• Workflow::with_dock starts a new scanning session at the saved dock (point_scan_runtime::new_engine). Workflow::take_dock_change reports a newly chosen dock once.
• New Adapter::remember hook, called after each switch action in scanning_runtime, saves the dock only when the chosen dock actually changed. The temporary move away from the pointer (moved) is never saved. A failed save logs a generic message and keeps the dock for the running app.
• resume_from also carries the chosen dock into the next scan, so the position holds within a run even if saving fails.
• scan_panel::Dock derives serde. It is not added to the synced scanner config, portable settings or Supabase preferences, so the synced format is unchanged.

Validation

• npm run lint: pass
• npm test: 295/295 vitest tests (13 files) and 12/12 node tests pass
• npm run build: pass
• cargo fmt --check: pass
• cargo clippy --all-targets -- -D warnings: pass
• cargo test: 761 passed, 0 failed, 5 ignored; config tests 7 passed
• New tests: panel_position::tests::a_saved_position_loads_back, panel_position::tests::missing_or_invalid_positions_use_the_default (temp dirs), point_workflow::tests::a_saved_panel_position_is_restored_and_only_chosen_positions_are_saved (no real input).
• npm run prediction-model was not run. It fails on this machine with an EPERM rename in .cache/prediction-neural, a local Windows lock unrelated to this change. The gitignored src-tauri/resources/prediction-model, src-tauri/resources/prediction-neural and src-tauri/binaries were copied from the main checkout for the build and tests.
• Not yet checked on a real device: move the panel, then restart scanning and the app.

Review

• Independent review of 00f4113 raised two low-severity findings: a failed save was counted as saved and never retried, and the file was not flushed before the rename. Both are fixed in 0999b73: the position is marked saved only after a successful write, so the next switch action retries, and the file is sync_alled before replace_file. Rust fmt, clippy and tests passed again (761 passed).
• A second independent review of 0999b73 reported no actionable findings.

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