chore(deps/ui): upgrade ratatui 0.29 and refresh gradients

Acceptance Criteria:
- Workspace builds against ratatui 0.29, crossterm 0.28.1, and tui-textarea 0.7 with palette support enabled
- Chat header context and usage gauges render with refreshed tailwind gradients
- Header layout uses the Flex API to balance top-row metadata across window widths

Test Notes:
- cargo test -p owlen-tui

Cargo.toml

@@ -34,9 +34,9 @@ futures = "0.3"
 futures-util = "0.3"
 
 # TUI framework
-ratatui = "0.28"
-crossterm = "0.28"
-tui-textarea = "0.6"
+ratatui = { version = "0.29", features = ["palette"] }
+crossterm = "0.28.1"
+tui-textarea = "0.7"
 
 # HTTP client and JSON handling
 reqwest = { version = "0.12", default-features = false, features = ["json", "stream", "rustls-tls"] }