feat(client): structured modules + tachyonfx animations

[Mayu-infinite]

Summary

Restructures the Rust client into clean, single-responsibility modules and adds
a set of tasteful terminal animations using tachyonfx (pinned to 0.16 for
ratatui 0.29 compatibility). Effects are adapted from junkdog's exabind
tech-demo techniques.

Structure

• app.rs — pure state + input/IPC logic (the old monolithic draw() is gone).
• ui.rs (new) — all layout + widget rendering in one place.
• effects.rs (new) — an Effects type owning every animation, with on_*
  triggers and render_* passes.
• main.rs — tracks per-frame elapsed time and drives effects at ~30 FPS.

Animations

Effect	Where	Trigger
Sweep + coalesce intro	whole screen	on launch
Header "breathing" glow	header	continuous
Thinking border glow (cyan)	transcript border	while assistant is busy
Message materialize (coalesce)	transcript	on new message
Status pulse	status bar	connect (green) / error (red)
Keystroke flash	prompt	on each keypress

Adapted from exabind's open_category (layered reveal), key_press (click
flash), and its LED-border idea (scoped to border cells via
CellFilter::Outer).

Verification

• cargo build — 0 errors
• cargo fmt --check + cargo clippy — clean
• cargo test — headless smoke test drives every effect over a real buffer for
  20 frames without panicking (1 passed)

Notes

• No functional/protocol changes to the IPC layer.
• Pre-existing IPC scaffolding dead_code warnings are unrelated to this PR.