GUI: "Browser Back/Forward" labels hide that they're what Finder needs

### What

The action picker offers "Back (Button 4)" (`MouseBack`) and "Browser Back" (`BrowserBack`). A user whose Back button is dead in Finder won't pick an action labelled "Browser" to fix a Finder problem — but ⌘[ is exactly what Finder honours, while button 4 is what it ignores. #1263's author was steered wrong by exactly this, and the `action.rs` rustdoc still sells `MouseBack` as the one "browsers and most apps interpret natively", which macOS reverses.

### Proposal

- Relabel "Browser Back/Forward" to something app-neutral — e.g. "Navigation Back (⌘[)" — via the `actions.browser_back` / `actions.browser_forward` locale keys.
- Fix the `action.rs` doc comments to state the macOS reality: buttons 4/5 navigate in Chromium/Firefox-family apps only; ⌘[ / ⌘] is honoured by Finder, Safari, Chrome and Firefox alike.

The seeded per-app profiles in #1351 fix the out-of-the-box experience; anyone customizing bindings still faces the label.

Refs #1263

Find the actions.browser_back and actions.browser_forward locale entries and inspect action.rs's MouseBack and BrowserBack rustdoc. Run the relevant GUI or localization checks, then confirm the picker uses app-neutral labels and the comments describe macOS behavior accurately.

Written by the indexing model from the issue text.