refactor(io): set up file association once, not on every launch (#205)

Drop the every-launch (re)registration thread. File association is now set up
once via the first-launch prompt (Yes → register + set default; No → decline,
persisted as file_assoc_enabled) and managed afterwards by the FILEASSOC
command. set_default_app now registers the handler entries (.desktop / mime /
ProgIDs) itself before asking the OS to default to us, since boot no longer
does — so the default never points at a missing handler.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/update/mod.rs

@@ -2590,14 +2590,18 @@ impl OpenCADStudio {
                 Task::none()
             }
             Message::AssocPromptYes => {
+                self.file_assoc_enabled = true;
                 self.mark_assoc_prompted();
                 self.active_modal = None;
+                // set_default_app registers the handler first, then makes us the
+                // default — boot no longer does this automatically.
                 Task::perform(
                     crate::io::file_association::set_default_app(),
                     Message::AssocResult,
                 )
             }
             Message::AssocPromptNo => {
+                self.file_assoc_enabled = false;
                 self.mark_assoc_prompted();
                 self.active_modal = None;
                 Task::none()