Add basic tab group rendering for horizontal tabs (warpdotdev#12089)

## Description
Adds basic rendering for **horizontal tab groups** in the top tab bar,
on top of the existing vertical tab grouping data model. A group renders
as a single tab-bar slot containing a header (icon collage + name)
followed by its member tabs.

- Click the header → collapse/expand the group (collapsed shows just the
header).
- Double-click the header → rename inline.
- Right-click the header → open the group context menu (labels adapt to
"left/right" for horizontal layouts, "up/down/above/below" for
vertical).
- Right-click a member tab → standard tab menu, including "Remove from
group".

Dragging is out of scope for this PR — neither groups nor members can be
dragged yet.

All new behavior is gated behind `FeatureFlag::GroupedTabs`; when the
flag is off, the horizontal tab bar renders exactly as before.

## Changes
**`workspace/view.rs`**
- Added `TabBarSlot { Single | Group }` enum and a preprocessing pass in
`render_tab_bar_contents` that turns the flat tab list into a list of
slots (ungrouped tabs become `Single`, contiguous same-group runs
collapse into `Group`).
- Added `render_horizontal_tab_group` (container + member tabs) and
`render_horizontal_tab_group_header` (icon collage + name +
click/double-click/right-click handlers).
- Added `compute_group_member_kinds` — picks up to 4 distinct pane icons
for a group by reusing each member tab's "Summary pair" selection, so
the group icons are always a subset of what the member tabs would show
in vertical Summary.
- Added `render_group_member_icon_collage` — 1 or 2 icons reuse vertical
Summary's `Single`/`Pair` layout for visual parity; 3 or 4 fall back to
a corner collage. Designs for reference:
<img width="222" height="92" alt="Screenshot 2026-06-02 at 4 14 24 PM"
src="https://github.com/user-attachments/assets/53a04f14-6ff9-4385-8236-878ca8630947"
/>
<img width="193" height="80" alt="Screenshot 2026-06-02 at 4 14 52 PM"
src="https://github.com/user-attachments/assets/d0016562-10c3-4226-8828-ef16fb295511"
/>


- Added `select_unique_pane_kinds` — shared selection helper used by
both vertical Summary (`select_summary_pane_kind_icons`) and the
horizontal collage.
- Added per-group hover state map (`horizontal_tab_group_mouse_states`)
on `Workspace`.
- `tab_group_menu_items` now takes `is_vertical: bool` so labels adapt
to layout.

**`tab.rs`**
- `TabComponent` gained a `grouped_member` flag and
`.for_grouped_member()` builder. In that mode the tab skips side
dividers, paints an inset rounded highlight, and skips the `Draggable`
wrapper (member dragging is a follow-up).

**`workspace/view/vertical_tabs.rs`**
- `render_summary_pane_kind_icons` and `SummaryPaneKindIcons` are now
`pub(super)` and accept a `total_size` parameter so the horizontal
collage can reuse them at its own sizing.
- `select_summary_pane_kind_icons` now delegates to the shared
`select_unique_pane_kinds` helper.

## Linked Issue


https://linear.app/warpdotdev/issue/APP-4641/horizontal-tab-grouping-basic-rendering


## Testing
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### Screenshots / Videos

[Demo
Video](https://www.loom.com/share/fff045cca12841bab0abd6b9764d89b9)

Author: johnturcoo

app/src/tab.rs

@@ -764,6 +764,8 @@ pub struct TabComponent<'a> {
     ///     flicker), and
     ///   * does not act as its own draggable / drop target.
     for_drag_ghost: bool,
+    /// Set when rendered as a member of a horizontal tab group.
+    grouped_member: bool,
 }
 
 /// Structure that holds TabComponent styles.
@@ -924,6 +926,7 @@ impl<'a> TabComponent<'a> {
             is_drag_target,
             background_opacity,
             for_drag_ghost: false,
+            grouped_member: false,
         }
     }
 
@@ -934,6 +937,13 @@ impl<'a> TabComponent<'a> {
         self
     }
 
+    /// Marks this tab as a member of a horizontal tab group. See the
+    /// [`TabComponent`] `grouped_member` field for the rendering differences.
+    pub fn for_grouped_member(mut self) -> Self {
+        self.grouped_member = true;
+        self
+    }
+
     /// Returns the agent indicator for the focused session's active conversation,
     /// or `None` if there is no non-empty, non-passive conversation to display.
     /// When a shell command is long-running the status is overridden to
@@ -1452,9 +1462,13 @@ impl<'a> TabComponent<'a> {
                 )
                 .finish(),
             );
-            Container::new(flex_row.finish())
-                .with_horizontal_padding(8.)
-                .finish()
+            let mut container = Container::new(flex_row.finish()).with_horizontal_padding(8.);
+            // Pad inside the Stack so the close-button overlay (anchored to
+            // the Stack) stays vertically centered within the visible pill.
+            if self.grouped_member {
+                container = container.with_vertical_padding(5.);
+            }
+            container.finish()
         };
 
         let compact_icon = {
@@ -1591,6 +1605,19 @@ impl<'a> TabComponent<'a> {
         )
         .finish();
 
+        // Grouped member: inset rounded highlight, no side dividers, no
+        // drop target (members can't be dragged currently).
+        if self.grouped_member {
+            let highlight = Container::new(stack)
+                .with_background(background_color)
+                .with_corner_radius(CornerRadius::with_all(Radius::Pixels(8.0)))
+                .finish();
+            return Container::new(highlight)
+                .with_vertical_padding(3.)
+                .with_horizontal_padding(3.)
+                .finish();
+        }
+
         let mut tab = Container::new(stack)
             .with_vertical_padding(2.)
             .with_background(background_color);
@@ -1657,6 +1684,7 @@ impl UiComponent for TabComponent<'_> {
         let mouse_close_state = self.tab.close_mouse_state.clone();
         // Capture before `self` is moved into the Hoverable closure below.
         let for_drag_ghost = self.for_drag_ghost;
+        let grouped_member = self.grouped_member;
 
         // Extract values before moving self into closure
         let tooltip_text = self.tooltip_message.clone();
@@ -1849,6 +1877,10 @@ impl UiComponent for TabComponent<'_> {
         // position cache, breaking `tab_insertion_index_for_cursor`.
         let full_tab: Box<dyn Element> = if for_drag_ghost {
             constrained_tab
+        } else if grouped_member {
+            // Skipping dragging within a group for now.
+            // TODO(johnturcoo) support dragging tabs within a group.
+            SavePosition::new(constrained_tab, &tab_position_id(tab_index)).finish()
         } else {
             let draggable = Draggable::new(draggable_state, constrained_tab)
                 .on_drag_start(|ctx, _, _| ctx.dispatch_typed_action(WorkspaceAction::StartTabDrag))