feat(viewport): RMB=Enter option + timed right-click

Adds a configurable right-mouse-button behavior without disturbing orbit
or the context menu.

- New persisted setting `rmb_enter` (command `RMBENTER [ON|OFF]`, default
  off): when on, a right-click while a command is active acts as Enter
  (commit/close); when idle it still opens the context menu. Right-drag
  always orbits, so nothing is lost.
- Right-click vs orbit is now time-based: orbit engages only after the
  button is held >=150ms and the pointer moves, so a quick right-click is
  no longer swallowed by slight pointer jitter (was an 8px distance gate).
- The right-click Repeat menu now lists commands from every source
  (command line, ribbon, context menu, shortcuts) via a unified
  recents list recorded at the dispatch choke point, instead of only
  typed commands.

Closes #185

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

src/scene/pick/selection_state.rs

@@ -24,6 +24,7 @@ pub struct SelectionState {
     pub left_dragging: bool,
     pub right_down: bool,
     pub right_press_pos: Option<Point>,
+    pub right_press_time: Option<Instant>,
     pub right_dragging: bool,
     pub right_last_pos: Option<Point>,
     pub middle_down: bool,