Replace per-block usage footer with persistent input status bar

Move token/cache/cost metrics from the expandable per-block usage button
into a persistent status bar rendered in the agent input area. This gives
always-visible feedback without requiring user interaction.

- Add render_session_status_bar to agent input showing context %, cache
  hit rate, and cost
- Persist cache/cost totals in ConversationUsageMetadata for session restore
- Remove render_usage_button, ToggleIsUsageFooterExpanded action, and
  UsageFooterToggled event
- Remove "Request tokens: --" debug block footer overlay
- Fix has_footer() to return false so blocks no longer reserve phantom
  footer space (root cause of padding/margin issue)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/terminal/model/block.rs

@@ -2063,7 +2063,7 @@ impl Block {
     }
 
     pub fn has_footer(&self) -> bool {
-        self.show_memory_stats
+        false
     }
 
     pub fn footer_top_padding(&self) -> Lines {