refactor: route GTK clipboard read/write through engine callbacks (#417)

[JDonaghy]

Summary

• Drops App.clipboard: Option<Box<dyn ClipboardProviderExt>> and the inline copypasta_ext setup in App::init. All clipboard work now goes through engine.clipboard_read / engine.clipboard_write — the same callback model the TUI already uses.
• 6 GTK call sites rewired: yank sync, paste prep, hover-popup copy, terminal copy, terminal paste, AI panel Ctrl-V.
• New helper setup_gtk_clipboard(&mut Engine) wires the provider once at startup, wrapped in Rc<RefCell<…>> so the shared-ref Fn callbacks can acquire the &mut self that ClipboardProviderExt::{get,set}_contents need.
• A future macOS (NSPasteboard) backend now installs its own callbacks with zero call-site changes. Closes Deduplicate clipboard read/write across backends #417.

Test plan

Built on macOS dev tree: cargo test --no-default-features --lib → 1963 passed; cargo fmt clean. GTK build / clippy can't run on this machine (glib-2.0 not installed). Pre-existing failures on develop (src/render.rs ContextMenuItem field bump, unused dir in core/engine/windows.rs:555) are unrelated.

Linux smoke checklist (no behavior change expected):

• cargo build succeeds (ignore the pre-existing render.rs / windows.rs failures on develop — separate issue).
• Yank in vimcode (yy), paste into another app — confirms sync_plus_register_to_clipboard → clipboard_write.
• Copy from another app, then p in normal mode — confirms needs_clipboard_for_paste → clipboard_read → prepare_paste_clipboard.
• Hover popup focused, press y (or Y, or Ctrl-C) — should show "Hover text copied" and paste outside.
• Integrated terminal: select + copy → paste outside; copy outside → paste into terminal.
• AI sidebar input focused, Ctrl-V — should insert the system clipboard text.

If any check fails, the regression is in setup_gtk_clipboard or one of the 6 rewired sites.

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