/diff viewer shows only leaf filename in tab bar — ambiguous for lib.rs, mod.rs, etc.

### Describe the feature or problem you'd like to solve

The `/diff` viewer's file navigator tab bar shows only the leaf filename (`lib.rs`, `mod.rs`) instead of the full or distinguishing path. In Rust projects (and other ecosystems with convention-based filenames), dozens of files share the same leaf name, making it impossible to tell which file is which.

### Proposed solution

Show the full relative path — or at minimum a disambiguated path suffix — in the `/diff` file navigator tab bar.

Currently the tab label is the *only* file identifier, with no tooltip or secondary path display. If two changed files share a basename (e.g., `crates/driver/src/lib.rs` and `crates/runtime/src/lib.rs`), both tabs show `lib.rs` with no way to distinguish them.

**Suggested approach:** Show the full relative path in the tab label, or use VS Code-style automatic path disambiguation — when two tabs share a filename, append enough parent directory segments to distinguish them (e.g., `driver/src/lib.rs` vs `runtime/src/lib.rs`).

### Example prompts or workflows

1. Open a Rust workspace with multiple crates, make changes in two different `lib.rs` files, run `/diff` — both tabs show `lib.rs`, no way to know which is which.
2. Same problem with `mod.rs`, `main.rs`, `build.rs` across crates in a workspace.

### Additional context

This is especially painful in Rust where `lib.rs`, `mod.rs`, `main.rs`, and `build.rs` are convention-based names repeated across every crate in a workspace. Go (`main.go`), Python (`__init__.py`), and Java (`Application.java`) have similar patterns.

Parti dalla barra delle schede del navigatore dei file di /diff e riproduci il problema con file modificati che condividono lo stesso nome di base, come crates/driver/src/lib.rs e crates/runtime/src/lib.rs. Individua il punto di ingresso esistente per le etichette delle schede e fai in modo che le etichette distinguano questi file mostrando i relativi percorsi completi o sufficientemente specifici; il lavoro è completato quando i nomi di base duplicati sono inequivocabili.

Scritto dal modello di indicizzazione a partire dal testo della issue.