Add Properties-panel Constraints/Parameters sections, a constraint-pill

value toggle, and redesigned constraint-ribbon icons

Properties panel:
- New "Parameters" section (shown with no selection) listing every named
  parameter with inline name/formula editing, add, and delete, backed by
  new PropValue::ParamRow/ParamAddRow and command_driver handlers
  (resolve_named_parameter_edit, on_prop_param_commit/_delete/_add_new).
  A fresh parameter's formula now defaults to "1", not "0" — a 0-length
  seed value is a genuine solver singularity for a Distance constraint
  driven by that parameter, with no defined direction to grow back out of.
- New "Constraints" section (shown for a single selected entity) listing
  every constraint touching it as a clickable PropValue::EntityLink that
  selects the linked entities in the viewport, danger-tinted when the
  constraint conflicts.

Constraint pills:
- New Options > General toggle, "Show values and parameter names on
  constraint markers" (on by default, persisted). Off, every pill shows
  just its bare glyph instead of glyph+driven-value/parameter-name — the
  glyph alone is enough to see a constraint is present, without the value
  text covering nearby geometry on a dense sketch.

Constraint-ribbon icons:
- Replaced all 18 unicode/letter glyph icons (including the plain "F",
  "L", "M", "S" letters) with proper SVGs under assets/icons/constrain/,
  matching the Modify section's two-color style (#B4B6B9 white / #6DB7ED
  blue, consistent stroke width). Tangent reuses the Draw section's
  white-arc-plus-blue-dots motif.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/settings.rs

@@ -244,6 +244,12 @@ pub struct UserSettings {
     /// graph keeps it synced regardless of this setting).
     #[serde(default)]
     pub write_dwg_native_constraints: bool,
+    /// When true (default), a sketch constraint's viewport pill shows its
+    /// glyph plus a driven value or named-parameter name. When false, every
+    /// pill shows just the bare glyph, so the value/name text doesn't cover
+    /// canvas detail on a dense sketch.
+    #[serde(default = "default_show_constraint_values")]
+    pub show_constraint_values: bool,
     /// Minutes between autosaves to a `.sv$` recovery file (SAVETIME command).
     /// 0 disables autosave.
     pub savetime_min: i32,
@@ -312,6 +318,10 @@ fn default_dimension_continue_mode() -> i16 {
     1
 }
 
+fn default_show_constraint_values() -> bool {
+    true
+}
+
 fn deserialize_clipromptlines<'de, D>(de: D) -> Result<i32, D::Error>
 where
     D: serde::Deserializer<'de>,
@@ -361,6 +371,7 @@ impl Default for UserSettings {
             backup_on_save: true,
             file_assoc_enabled: true,
             write_dwg_native_constraints: false,
+            show_constraint_values: true,
             savetime_min: 10,
             default_save_format: crate::io::DEFAULT_SAVE_FORMAT.to_string(),
             pick_add: true,