feat: add configurable “Open in” targets (settings UI, custom commands/args, ordering)

[Dimillian]

Motivation

• Provide a user-configurable version of the existing "Open in" menu so users can add arbitrary apps or commands with custom launch arguments and reorder the list, and make the file-preview and message link actions use that configuration.

Description

• Add a new OpenAppTarget type and persistent settings fields openAppTargets and selectedOpenAppId (frontend src/types.ts, backend src-tauri/src/types.rs) and expose default targets in src/features/app/constants.ts.
• Add a new Settings → "Open in" section (src/features/settings/components/SettingsView.tsx) to add/edit/remove targets, set app / command / args, choose a default, and reorder entries, plus supporting styles in src/styles/settings.css.
• Wire the configurable targets into UI and link handlers: OpenAppMenu, FilePreviewPopover, Messages and useFileLinkOpener now respect configured targets and selected default, and selection is synced to localStorage and saved in app settings.
• Extend the IPC/service and backend openWorkspaceIn/open_workspace_in to accept optional command and args so targets can launch via macOS open -a or arbitrary commands with args (src/services/tauri.ts, src-tauri/src/workspaces.rs).

Testing

• Ran lint with npm run lint and addressed warnings; lint completed successfully.
• Ran the test suite with npm run test (vitest) and all tests passed (39 files, 180 tests).
• Ran type checking with npm run typecheck (tsc --noEmit) and it completed with no errors.

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Codex Task

[Dimillian]

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