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/server/server_api/ai.rs

@@ -2292,6 +2292,10 @@ fn convert_usage_metadata(
         credits_spent_for_last_block: None,
         token_usage: vec![],
         tool_usage_metadata: Default::default(),
+        total_cache_read_tokens: 0,
+        total_cache_write_tokens: 0,
+        total_cache_miss_tokens: 0,
+        total_cost_cents: 0.0,
     }
 }