fix(ctx-menu): anchor right-click menu under cursor, share overlay infra

The right-click context menu was being placed via window-relative
coordinates while the cursor position it anchored to was canvas-relative,
so the menu drifted away from the cursor by the size of the surrounding
ribbon / sidebar / status bar.

Move the menu inside `viewport_stack` so its anchor resolves in the same
coordinate space the cursor was captured in. Introduce
`position_canvas_overlay(anchor, panel)` — wraps `panel` in a column+row
of fixed-width spacers and `iced::widget::opaque(panel)` so events on the
menu itself don't fall through to the viewport mouse area underneath.
The multi-functional grip popup is rewritten to use the same helper.

Outside-click dismiss now mirrors the grip popup: `ViewportLeftPress`
checks `selection.context_menu.take()` at the top and returns early
when it was set, so a press that reaches the viewport mouse area
(i.e. outside the opaque panel) closes the menu without running the
rest of the left-click handler. The redundant `context_menu = None`
later in the handler and the now-unused `Message::ViewportContextMenuClose`
are removed.

Two related issues fixed in the same pass:

* The right-drag threshold for orbit was 3 px squared, small enough that
  normal hand jitter between a right-button press and release was
  promoting a click to a drag and suppressing the menu on release.
  Bumped to 8 px squared.
* `Message::ViewportExit` was clearing `context_menu`. With the menu now
  living above the viewport mouse area, the opaque panel capturing the
  cursor fires the underlying area's exit handler — which closed the
  menu the instant it opened. Stop clearing the menu there; outside-click
  dismiss is enough.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/mod.rs

@@ -627,8 +627,6 @@ pub enum Message {
     // ── About window ────────────────────────────────────────────────────
     AboutOpen,
     AboutCopyInfo,
-    /// Close the viewport right-click context menu without performing any action.
-    ViewportContextMenuClose,
     /// The user clicked the title-bar ✕ (fires before the window closes).
     WindowCloseRequested(window::Id),
     /// A window was fully closed (fires after `window::close()` is called).