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>

crates/graphql/src/api/queries/get_conversation_usage.rs

@@ -171,6 +171,10 @@ impl From<&ConversationUsageMetadata> for persistence::model::ConversationUsageM
             credits_spent_for_last_block: None,
             token_usage: convert_token_usage(&gql.warp_token_usage, &gql.byok_token_usage),
             tool_usage_metadata: (&gql.tool_usage_metadata).into(),
+            total_cache_read_tokens: 0,
+            total_cache_write_tokens: 0,
+            total_cache_miss_tokens: 0,
+            total_cost_cents: 0.0,
         }
     }
 }