Add native file dialogs for workspace and export paths

A 📂 button beside each path field opens a native picker: a folder
chooser for the workspace and an .odt save dialog for the export (which
appends the extension if omitted). Uses rfd with the XDG Desktop Portal
backend, so there is no GTK build dependency and no new shared library
at runtime.

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

README.md

@@ -51,8 +51,9 @@ Terminal=false
 
 ## Use
 
-1. Set the **Workspace** path in the top bar (default: `~/Manuscript`) and press
-   **Open**. The directory is created if it does not exist.
+1. Set the **Workspace** path in the top bar (default: `~/Manuscript`) — type it,
+   or click **📂** to pick a folder in a native file browser — and press **Open**.
+   The directory is created if it does not exist.
 2. Press **Init git** once to make the workspace a git repository. To sync with
    another machine, add a remote yourself:
    ```sh
@@ -69,7 +70,8 @@ Terminal=false
    blank falls back to a `# Title:` line, then the file's first level-1 heading
    (`# ...`), then the file name. Titles are saved to `titles.json` and sync with
    git.
-5. Set the **Export** path and press **Export ODT** to produce the concatenated
+5. Set the **Export** path (type it, or click **📂** to choose the `.odt` file in a
+   native save dialog) and press **Export ODT** to produce the concatenated
    document. Each file becomes a chapter headed by its title, optionally followed
    by a caption from its `# Slug:` line.