refactor: split app/streams.rs by streaming concern

streams.rs (639 lines) was a single impl App mixing object-watch,
pod-log streaming, exec/terminal sessions, port-forwarding, and the
YAML explanation dialog. Split into streams/{object_watch,logs,exec,
port_forward,yaml_explanation}.rs, one impl App block per concern
(mirrors the methods.rs split from earlier in this refactor pass).

Relocated methods are called from handler.rs/methods.rs (siblings, not
descendants), so visibility moves from pub(super) to pub(crate)
uniformly, matching the methods.rs precedent. Free helper functions
(terminal_container_dropdown/terminal_view/terminal_window) stay
private in exec.rs, the only file that uses them.

Verified: cargo check/fmt/clippy/test all clean, plus a
`cargo run --bin aetheris` smoke run — process starts and stays alive
with no panic/crash output.

Author: leandromqrs