chore: clear compiler warnings

Remove accidental dead code (unused imports, the superseded paste tool()
helper, a never-read TORIENT handles field, a duplicate let binding, an
unneeded mut) and mark intentional-but-unwired scaffolding with
allow(dead_code): the web-only save_to_bytes, the plugin loadable() probe,
the planned PICK_HIGHLIGHT colours, the DynFieldSpec label, and the layout
ribbon helpers. Native build is now warning-clean apart from the upstream
nom/quick-xml future-incompat notices.

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

src/app/update.rs

@@ -1677,7 +1677,6 @@ impl OpenCADStudio {
                 // buffer to the same submit path so typing a command name (or
                 // an option keyword like "R") and pressing Enter works without
                 // first clicking into the command line (issue #99).
-                let i = self.active_tab;
                 if !self.command_line.input.trim().is_empty() {
                     return self.update(Message::CommandSubmit);
                 }