Add per-tempfile Open button and action to launch files

[multiplex55]

Motivation

• Allow users to open individual temp files directly from the Tempfiles dashboard widget instead of only clearing or opening the containing folder.
• Introduce a dedicated action prefix for opening a specific file so the launcher can route it to OS file opening logic.
• Ensure the UI tooltip clarifies the button opens the file itself and not the containing folder.

Description

• Added an Open small button per tempfile row and an open_action constructor in src/dashboard/widgets/tempfiles.rs that emits tempfile:open:<path> when clicked.
• Introduced pub fn open_file(path: &str) in src/actions/tempfiles.rs which calls open::that(Path::new(path)) to let the OS open the file.
• Extended ActionKind in src/launcher.rs with TempfileOpenFile(&str), updated parse_action_kind to parse the tempfile:open:<path> prefix, and wired launch_action to call tempfiles::open_file(path).
• Updated the button tooltip to "Open file (not folder)" to make the behavior explicit to users.

Testing

• No automated tests were run for this change.
• No CI or cargo test/cargo build runs were executed as part of this patch application.

---

Codex Task