feat(gui): open log line via configurable editor command (template {file}/{line})

refactor(settings): defaults carried by SettingsItem, getters/setters via _pref
refactor(pref-win): declarative Field table + _FIELD bridge + merged file pickers

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

DESIGN.md

@@ -323,6 +323,7 @@ The `testium_assist` editor extension is a thin LSP client that spawns `testium
 Both Flatpak and AppImage export `TESTIUM_VERSION` from a launcher (Flatpak: launcher script in `org.testium.Testium.yaml`; AppImage: `runtime.env` in `AppImageBuilder.yml`). `get_testium_version()` checks `/.flatpak-info` / `APPIMAGE` and reads `TESTIUM_VERSION` rather than relying on package metadata or repo introspection.
 
 ## Recent fixes / notable changes
+- Open-log-at-line (GUI): double-click on a tree item opened the log via a hardcoded `code -g {file}:{line}` (broke in Flatpak where `code` is absent). Now driven by a configurable `editor_cmd` preference (placeholders `{file}`/`{line}`, default `code -g {file}:{line}`); the argv is built by `shlex.split` then per-token `.format` (paths with spaces stay one token), wrapped by `bins.host_console_command()` for Flatpak host-spawn, with a `host_open_path`/`openUrl` fallback (no line) when empty or failing. Settings/pref refactor alongside: `SettingsItem` carries its default (single source of truth), trivial getters/setters collapse to `_pref(item)` bindings, the pref window's `elements` dict becomes a `Field(key, type, widget)` table with a per-type `_FIELD` read/write bridge, and the four file-picker slots fold into `_pick_dir`/`_pick_file`. (Also fixed a latent default mismatch: `report_path` defaulted to `$(home)` in the property but `$(test_directory)` in the pref window; unified to `$(test_directory)`.)
 - Show Results (GUI): the toolbar action stays enabled during a run (the log grows live, so it is useful mid-test), not just after. In Flatpak `QDesktopServices.openUrl` routes through the OpenURI portal and often opens no editor for a `.log`; `bins.host_open_path()` now spawns `xdg-open` on the host via `flatpak-spawn --host` (returns False outside Flatpak so the caller falls back to `openUrl`).
 - Test-tree search (GUI): a Ctrl+F find bar highlights + navigates matching items, with Name/Type/Doc field checkboxes. Search modifications run under `blockSignals` (else `setBackground`→`itemChanged`→`on_testChecked` storms the controller), and the search/run highlights share one flag-driven `_refresh_highlight()` (run > search > default) so overlapping layers never leave a stale colour. See "## Test-tree search (GUI)".
 - `pytest` item: pytest analogue of `unittest`, but runs on the **host interpreter in a subprocess** (`bins.python_bin()`, like `py_func`) so it works across every packaging channel. A stdlib-only pytest plugin streams collected node-ids + per-test results back over stdout via sentinels; each test becomes a child item with its own PASS/FAIL/SKIP, duration and failure message. Params: `test_file`, `test_method`. Validation item: `test/validation/items/pytest/` (the validation venv now pip-installs `pytest`). See "### `pytest` item".