feat(ui): migrate Text Style and Multiline Style to modals (Plan B)

Move the Text Style and Multiline Style editors off OS windows onto the
shared `active_modal` / `modal_content` path. Their open handlers set
`active_modal` (keeping the existing buffer/stage setup); construction
moves from the daemon `view` dispatch into `modal_content`.

The remaining style editors (Table, MLeader, Dimension) still use OS
windows for now — they carry a nested colour picker that needs its own
handling. Native + web build; 75 tests pass.

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

src/app/mod.rs

@@ -601,6 +601,11 @@ pub enum ModalKind {
     PageSetup,
     LayoutManager,
     Plotstyle,
+    TextStyle,
+    TableStyle,
+    MlStyle,
+    MLeaderStyle,
+    DimStyle,
 }
 
 /// Identifies a DimStyle field that can be edited in the dialog.