feat: redesign Todos tab — grouping, collapsible headers, cleaned columns

Implements the approved mockup from the UX debate report:

- new DisplayRow::TodoHeader variant: collapsible group headers, toggled
  by Enter/Space, tracked in HubState.todo_group_collapsed
- [g] cycles grouping: status (default) -> project -> repo -> none
- [s] cycles which statuses are included before grouping: active
  (open+in-progress, default) -> open -> done -> all
- table: dropped SRC/EST (near-always empty), code-scan todos show a dim
  'no status icon' marker instead of a separate source column, project
  name colored by the project's own color (was DarkGray, wasting the most
  useful visual identifier in the tool), age now tracks updated_at not
  created_at ('what's moving' beats 'what's old')
- right panel: added Repo/Difficulty/Estimate, previously only visible in
  the table despite the panel having more room for them
- block title shows live counts + current view/group/code-scan state

Simplified vs. the literal ASCII mockup: [s] filters the underlying set
before grouping, so e.g. the default 'active' view omits Done/Cancelled
groups entirely rather than always showing them collapsed - a simpler,
more internally consistent model. Flagged for foksiu to request the exact
mockup behavior if he wants it.

Verified via pyte: group/status-view cycling, collapse/expand (arrow flips
and child rows appear/disappear), right panel on header vs. todo row.

src/hub/render.rs

@@ -60,10 +60,13 @@ pub static HELP_LINES: &[&str] = &[
     " p            Project menu (create / assign / rename / delete)",
     " +            New todo",
     " e            Edit todo (Todos tab)",
-    " Space        (Todos tab) Cycle todo status",
+    " Space        (Todos tab) Cycle todo status, or expand/collapse a group",
+    " Enter        (Todos tab) Expand/collapse a group header",
+    " s            (Todos tab) Cycle status view: active → open → done → all",
+    " g            (Todos tab) Cycle grouping: status → project → repo → none",
+    " c / X        (Todos tab) Toggle code-scan todos",
     " D / Del      Delete todo or project",
     " T            Scan repos for TODO/FIXME/HACK → import to Todos",
-    " X            (Todos tab) Toggle code-scan todos",
     "",
     "── Help ──────────────────────────────────────",
     " ?            Show this help",
@@ -165,7 +168,16 @@ pub fn render_hub(f: &mut Frame, state: &mut HubState) {
             .constraints([Constraint::Percentage(73), Constraint::Percentage(27)])
             .split(outer[1]);
 
-        let left_blk  = Block::default().borders(Borders::ALL);
+        let mut left_blk = Block::default().borders(Borders::ALL);
+        if state.tab == Tab::Todos {
+            let in_progress = state.todos.iter().filter(|t| t.status == crate::todos::TodoStatus::InProgress).count();
+            let open = state.todos.iter().filter(|t| t.status == crate::todos::TodoStatus::Open).count();
+            left_blk = left_blk.title(format!(
+                " todos · {} in progress · {} open · [s]tatus:{} · [g]roup:{} · [c]ode-scan:{} ",
+                in_progress, open, state.todo_status_view.label(), state.todo_group_by.label(),
+                if state.show_code_todos { "on" } else { "off" },
+            ));
+        }
         let left_inner = left_blk.inner(cols[0]);
         f.render_widget(left_blk, cols[0]);
         state.visible_rows = left_inner.height.saturating_sub(1) as usize;