feat(styles): unify the five style managers behind a shared frame

The text/dimension/table/multileader/multiline managers were hand-copied,
so their list CRUD kept drifting and breaking: Dimension's New created
nothing (#73), new text styles never reached the ribbon dropdown (#74),
and several were added with a NULL handle (dropped on DWG save, #67).

Collapse the duplicated parts into three shared pieces and leave only the
per-manager property editor bespoke:

- app/style_ops.rs: one StyleKind-dispatched CRUD layer (new / copy /
  delete / rename) handling handle allocation, ribbon sync, Table re-key
  and name-reference rewrites in a single place.
- ui/style_manager.rs: the window scaffold — uniform toolbar (New / Copy /
  Delete on the left, Set Current + Apply on the right), style list and
  chrome. Each manager now only builds its editor.
- ui/style_list.rs: the list row — single click selects, double click
  starts an inline rename.

New capabilities, now consistent across all five:
- Copy duplicates the selected style.
- Double-click a name to rename it inline.
- Right side is always Set Current + Apply (added Table set-current;
  MLine apply is a documented placeholder until its editor is editable).

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

src/ui/style_list.rs

@@ -0,0 +1,43 @@
+//! Shared style-manager list row.
+//!
+//! Every style manager renders the same left-hand list of style names where a
+//! single click selects and a double click renames inline. Only the row's
+//! button style differs per manager, so it is passed in.
+
+use crate::app::{Message, StyleKind};
+use iced::widget::button::{Status, Style};
+use iced::widget::{button, mouse_area, text, text_input};
+use iced::{Element, Fill, Theme};
+
+/// One row of the style list. Renders an editable `text_input` when `name` is
+/// the style being renamed (`rename_active`), otherwise a selectable button
+/// wrapped in a `mouse_area` whose double click starts the rename.
+pub fn item<'a>(
+    name: &str,
+    label: String,
+    kind: StyleKind,
+    on_select: Message,
+    rename_active: Option<&str>,
+    rename_buf: &'a str,
+    style: impl Fn(&Theme, Status) -> Style + 'a,
+) -> Element<'a, Message> {
+    if rename_active == Some(name) {
+        text_input("", rename_buf)
+            .on_input(Message::StyleRenameEdit)
+            .on_submit(Message::StyleRenameCommit(kind))
+            .size(11)
+            .padding([4, 8])
+            .width(Fill)
+            .into()
+    } else {
+        mouse_area(
+            button(text(label).size(11))
+                .on_press(on_select)
+                .style(style)
+                .padding([4, 8])
+                .width(Fill),
+        )
+        .on_double_click(Message::StyleRenameStart(kind, name.to_string()))
+        .into()
+    }
+}