Tell the user the badge is a button, and refresh the DoF when one is deleted

A glyph reads as decoration until something says otherwise, so the badges shipped
last commit were discoverable only by accident. Two places now say it, chosen because
they are where the eye already is:

- the moment of applying, which is the one the user is watching ("Applied constraint ·
  its badge is on the sketch — click the badge to remove it"). The hint line could not
  carry this alone: it only refreshes when the (mode, step, picks) tuple changes, and
  applying a constraint changes none of them.
- the Select-mode hint line, appended only while the live sketch actually holds a
  constraint, so it never advertises a badge that is not on screen.

Also fixes what the previous commit got wrong: remove_constraint_near solved through
solve_sketch_entities directly, which relaxes the geometry but leaves m_dof and the
per-entity conflict flags untouched and never fires on_solve_state. Deleting a
constraint therefore left the DoF readout describing the system as it was BEFORE the
deletion, and any red over-constrained tint stranded on screen. It goes through
resolve_live() now, the same path every other live edit uses.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011FbJKJAJxxkhDTs9XdZzKA

src/slic3r/GUI/CAD/DesignCanvas.cpp

@@ -1489,6 +1489,11 @@ const std::vector<SketchEntity>& DesignCanvas::sketch_entities() const
     return m_sketch_tool.entities();
 }
 
+int DesignCanvas::sketch_constraint_count() const
+{
+    return int(m_sketch_tool.constraints().size());
+}
+
 bool DesignCanvas::try_add_sketch_constraints(const std::vector<SketchEntityConstraintDef>& defs)
 {
     return m_sketch_tool.try_add_constraints(defs);