export_webview

The extension should provide a single export webview for exporting the requirement graph in various formats ( json_export, csv_export, html_export, and later formats such as pdf ).
This generalizes the current HTML-only html_export_form: format choice is primary, and format-specific options ( e. g. HTML runtime mode, template, page families ) appear when that format is selected.
Shared options: scope, output folder, and export name; defaults persist to application_memory as `export_settings.json` under `<workspace>/.reqlan/`.
While an export runs, the form should show a loading state with live progress ( phase message and, when known, completed / total or percent ) so the user can see HTML ( and other formats ) being built — not only a disabled button.
The user should be able to open it from:
- a link / control in the main ideas_summary chrome ( status bar or header )
- the command_palette

Status: partial

Interactive page: export_webview