Reuse the post view dialog and WebView instead of rebuilding it each time

[garo-pro]

Reuse the post view dialog and WebView instead of rebuilding it each time

Fixes #164

Problem

show_post_view_dialog (src/ui/dialogs/post_view.rs) created a new Dialog and WebView on every call and destroyed them when the dialog closed. Each time a user viewed a post, WebView2 had to be initialized from scratch. That made opening a post slow, and it brought back the first-launch layout problem on every open: lines were shorter than the user's NVDA/Windows text settings called for.

Change

The dialog, its WebView, and the Boost/Favorite buttons are now built once and cached in a thread_local. Each view only does the following:

• sets the dialog title,
• sets the Boost/Unboost and Favorite/Unfavorite labels from the post's current state,
• calls set_page with the new content.

The script injection (Escape to close, link handling) stays in the on_loaded handler. It is registered once and runs after every set_page, so behavior is unchanged. The dialog is no longer destroyed after show_modal. It lives for the life of the app, and wx tears it down with its parent frame on exit.

Trade-offs

• A WebView2 instance now stays alive after the first post view, which uses more memory than before. This is the cost of not reinitializing it each time.
• The cached dialog is parented to the frame passed on first use. That is always the main frame today.

Testing

• cargo clippy and cargo test pass, and CI is green on my fork.
• Tested manually: the post view now opens without the WebView2 reload delay after the first time.