fix(tui): force full repaint on theme switch to prevent stale sidebar colors

When switching themes, ratatui's incremental diff engine may miss
color-only changes in sidebar cells that were rendered with
theme-resolved UiTheme fields rather than palette constants routed
through the backend remap layer.  This manifests as the sidebar
retaining the previous theme's colors until a window resize or
conversation turn triggers a full repaint.

Add a force_next_full_repaint flag on App that is set whenever a
theme or background_color ConfigUpdated event is processed.  The
main render loop merges this into the existing force_terminal_repaint
mechanism, which clears the terminal and redraws every cell.

crates/tui/src/tui/app.rs

@@ -1504,6 +1504,11 @@ pub struct App {
     pub session_started_at: chrono::DateTime<chrono::Utc>,
     /// Whether the UI needs to be redrawn.
     pub needs_redraw: bool,
+    /// When true, the next draw will be a full repaint (terminal clear +
+    /// all cells redrawn) instead of a ratatui incremental diff. Used by
+    /// theme switches where the diff engine may miss color-only changes
+    /// in sidebar cells that were previously rendered with palette constants.
+    pub force_next_full_repaint: bool,
     /// When the current thinking block started (for duration tracking).
     pub thinking_started_at: Option<Instant>,
     /// Whether context compaction is currently in progress.
@@ -2094,6 +2099,7 @@ impl App {
             decision_card: None,
             session_started_at: chrono::Utc::now(),
             needs_redraw: true,
+            force_next_full_repaint: false,
             thinking_started_at: None,
             is_compacting: false,
             is_purging: false,

crates/tui/src/tui/ui.rs

@@ -2383,6 +2383,12 @@ async fn run_event_loop(
         } else {
             None
         };
+        // Merge the per-app full-repaint hint (set by theme switches)
+        // into the loop-level flag before the draw decision.
+        if app.force_next_full_repaint {
+            force_terminal_repaint = true;
+            app.force_next_full_repaint = false;
+        }
         if app.needs_redraw && draw_wait.is_none() {
             let was_full_repaint = force_terminal_repaint;
             draw_app_frame_inner(terminal, app, force_terminal_repaint)?;
@@ -6807,6 +6813,19 @@ async fn handle_view_events(
                 persist,
             } => {
                 let result = commands::set_config_value(app, &key, &value, persist);
+                // Theme / background changes require a full terminal repaint
+                // because ratatui's incremental diff may miss color-only
+                // changes in cells that were rendered with theme-resolved
+                // colors (sidebar panels) rather than palette constants that
+                // go through the backend remap layer.  A full repaint
+                // (terminal clear + all cells redrawn) guarantees every cell
+                // picks up the new theme immediately.
+                if matches!(
+                    key.as_str(),
+                    "theme" | "ui_theme" | "background_color" | "background" | "bg"
+                ) {
+                    app.force_next_full_repaint = true;
+                }
                 // Only surface the "key = value" confirmation when the
                 // change is being persisted. Live-preview events
                 // (`persist: false`, e.g. arrow keys in the theme picker)