[Security]: RUSTSEC-2026-0253: lru 0.12.5 in Cargo.lock is unsound (use-after-free via LruCache eviction)

### Summary

`lru` 0.12.5 — the version locked in `Cargo.lock` on `main`, pulled in
by `ratatui 0.29` — is covered by RUSTSEC-2026-0253 (later advisories
reference RUSTSEC-2026-0002): `LruCache::pop` is not panic-safe, and an
eviction that panics can leave the cache in an inconsistent state,
leading to a use-after-free or double free. The advisory is fixed in
`lru` >= 0.18.2, which `ratatui 0.29` cannot satisfy because it pins
`lru ^0.12`.

### Affected dependency chain

- `ratatui 0.29.0` (locked in `Cargo.lock`, CLI TUI startup path)
- `-> lru 0.12.5` (locked; affected range)

### Reproduction (from a clean checkout)

1. `git clone` the repository and check out `main`.
2. Run `cargo audit` (or `cargo deny check advisories`).
3. Observed: `lru` flagged as unsound at 0.12.5 (RUSTSEC-2026-0253),
with the advisory requiring `lru >= 0.18.2`.

### Suggested fix

Upgrade `ratatui` from 0.29 to 0.30 (whose `ratatui-core` requires
`lru ^0.18`, resolving the advisory), relax the `bitflags` pin from
`=2.11.1` to `^2` as required by `ratatui-core 0.1.2`, and carry the
small `Backend` trait adaptation in the CLI startup loop. I have a patch
ready and will open a PR referencing this issue.

Start with Cargo.lock and the dependency declarations that pull in ratatui 0.29, then run cargo audit or cargo deny check advisories from a clean checkout. Upgrade the dependency chain, inspect the CLI startup loop for the required Backend trait adaptation, and confirm the advisory is no longer reported and the project checks pass.

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