v0.2.4 — TUI update banner + end-to-end broadcast pipeline

[ungaro]

Two TUI additions. Backward-compatible — no CLI flag changes, no TxBundle shape changes.

Part A — Persistent TUI update banner

• Reads version_check::cached_latest() once at App::new (no per-frame disk hit). When Some(latest), a thin yellow row renders above the tabs bar: ▲ nest v{latest} is available — run \nest update` (Esc to dismiss)`.
• Top-level layout becomes a conditional 4-row vs 3-row split.
• Esc dismisses the banner for the session (only as a fall-through — existing Esc handlers like "close modal" / "back to Vaults from Deposit/Withdraw" still take precedence).

Part B — TUI broadcast pipeline (Deposit / Withdraw)

End-to-end submit flow with a safety-default confirm modal.

Submit-state machine (per form):
`Idle → BuildingBundle → AwaitingConfirm(TxBundle) → Broadcasting{bundle, progress: Vec} → Done{receipts: Vec} / Failed{error}`

Pipeline:

1. Enter on the Submit field → pre-flight (all fields, private key present) → spawn mint_build_tx / redeem_build_tx task → BuildingBundle.
2. Bundle returned → AwaitingConfirm(bundle) → form body shows per-tx preview lines ([i/N] {label} to: {short} value: {decimal-wei or —}); confirm modal opens.
3. Confirm modal: items are [Cancel, Broadcast]. Cancel is items[0] so Modal::new's default selection lands on it — single-press Enter is cancel; the user must ↑ then Enter to broadcast (deliberate, irreversible action).
4. Broadcast chosen → spawn one task per bundle, run each BundleTx as its own single-tx execute_bundle so progress streams in real time. mpsc messages: DepositSubmitProgress(idx, TxProgress::{Pending|Confirmed(TxResult)|Failed(err)}) — drain_fetches mutates state.
5. All-confirmed → Done{receipts} → form body replaced with one row per tx: ✓ [i/N] {label} {tx_hash} block N → {explorer_url} (explorer URL already populated by execute_bundle).
6. Any failed → Failed{error} → form body replaced with red error message.
7. Esc from Done/Failed → back to Idle, amount/shares cleared.

Withdraw: same shape via redeem_build_tx. Instant / Claim modes still emit the v0.2.3 CLI hint — wiring those is v0.2.5.

What's NOT in this release (deliberate)

• Withdraw Instant / Claim mode broadcast — they still print the CLI hint. v0.2.5.
• Bridge broadcast from the TUI — Bridge has no tab today. Future.
• Polling refresh of cached_latest() mid-session — the banner reflects the value at startup; new manifests show up next launch.

Verification

• cargo test — 69 passed (61 + 8 new): banner state, banner Esc fall-through, Cancel-first invariant, build→AwaitingConfirm + modal open, progress→Done transition, progress→Failed transition, formatted result line, cached_latest initial value.
• cargo clippy --all-targets -- -D warnings clean.
• cargo fmt -- --check clean.