Expose flexible panel settings in settings UI (#52946)

Release Notes:

- Added controls to the settings UI for whether the terminal and agent
panels use flexible width.

crates/settings_ui/src/page_data.rs

@@ -4990,7 +4990,7 @@ fn panels_page() -> SettingsPage {
         ]
     }
 
-    fn terminal_panel_section() -> [SettingsPageItem; 3] {
+    fn terminal_panel_section() -> [SettingsPageItem; 4] {
         [
             SettingsPageItem::SectionHeader("Terminal Panel"),
             SettingsPageItem::SettingItem(SettingItem {
@@ -5006,6 +5006,19 @@ fn panels_page() -> SettingsPage {
                 metadata: None,
                 files: USER,
             }),
+            SettingsPageItem::SettingItem(SettingItem {
+                title: "Terminal Panel Flexible Sizing",
+                description: "Whether the terminal panel should use flexible (proportional) sizing when docked to the left or right.",
+                field: Box::new(SettingField {
+                    json_path: Some("terminal.flexible"),
+                    pick: |settings_content| settings_content.terminal.as_ref()?.flexible.as_ref(),
+                    write: |settings_content, value| {
+                        settings_content.terminal.get_or_insert_default().flexible = value;
+                    },
+                }),
+                metadata: None,
+                files: USER,
+            }),
             SettingsPageItem::SettingItem(SettingItem {
                 title: "Show Count Badge",
                 description: "Show a badge on the terminal panel icon with the count of open terminals.",
@@ -5666,7 +5679,7 @@ fn panels_page() -> SettingsPage {
         ]
     }
 
-    fn agent_panel_section() -> [SettingsPageItem; 5] {
+    fn agent_panel_section() -> [SettingsPageItem; 6] {
         [
             SettingsPageItem::SectionHeader("Agent Panel"),
             SettingsPageItem::SettingItem(SettingItem {
@@ -5695,6 +5708,19 @@ fn panels_page() -> SettingsPage {
                 metadata: None,
                 files: USER,
             }),
+            SettingsPageItem::SettingItem(SettingItem {
+                title: "Agent Panel Flexible Sizing",
+                description: "Whether the agent panel should use flexible (proportional) sizing when docked to the left or right.",
+                field: Box::new(SettingField {
+                    json_path: Some("agent.flexible"),
+                    pick: |settings_content| settings_content.agent.as_ref()?.flexible.as_ref(),
+                    write: |settings_content, value| {
+                        settings_content.agent.get_or_insert_default().flexible = value;
+                    },
+                }),
+                metadata: None,
+                files: USER,
+            }),
             SettingsPageItem::SettingItem(SettingItem {
                 title: "Agent Panel Default Width",
                 description: "Default width when the agent panel is docked to the left or right.",