PgUp/PgDn should page the focused view

> **Relation to #1707:** that issue asks for a *new* binding (`ctrl`/`shift`/`alt` + pgup) to scroll the *main panel* by pages. This one is narrower and different: leave the main panel alone, and make the *existing, unmodified* PgUp/PgDn keys page the *focused* view, so they match `,`/`.`. It is a consistency fix to a key that already exists, not a new feature. The idea was raised inside #1707 by @jesseduffield ([comment](https://github.com/jesseduffield/lazygit/issues/1707#issuecomment-2864612011)) and @stefanhaller ([comment](https://github.com/jesseduffield/lazygit/issues/1707#issuecomment-2865372044)) in May 2025 but was never split out, so it has no issue of its own. Happy to have this folded back into #1707 if you'd rather keep it in one place.

### Is your feature request related to a problem? Please describe.

In v0.64.0, plain PgUp/PgDn are bound to `scrollUpMain`/`scrollDownMain`:

```yaml
scrollUpMain:
scrollDownMain:
```

So they move the main panel by `gui.scrollHeight` 2 lines by default. That surprises new users since the keys not only behave in ways that are unexpected, but it's the only view where PgUp/PgDn don't follow lazygit's own pattern.

| Key | Mirrors | Acts on |
|---|---|---|
| ↑ / ↓ | `k` / `j` | focused view |
| Home / End | `<` / `>` | focused view |
| PgUp / PgDn | — | **main panel, 2 lines** |

``/`` are already going to the top and bottom. ``/`` are already `prevItem`/`nextItem`. Every neighbour of PgUp/PgDn is doing the equivalent action on the focused view. PgUp/PgDn is the exception.

### Describe the solution you'd like

Make PgUp/PgDn the alt-bindings for `prevPage`/`nextPage`, exactly as Home/End are for `gotoTop`/`gotoBottom` and drop ``/`` from `scrollUpMain`/`scrollDownMain`.

Since v0.50 the main view is focusable with 0, so this loses no ability: focus the main view and PgUp/PgDn page the diff, which is what most people pressing those keys wanted anyway.

**On backwards compatibility** — this is the objection @jesseduffield raised in #1707, and I think it's understandable, it will be inconvenient for users used to the old behavior. I still think it should be implemented for the following:

- Main-panel line-scrolling keeps two bindings, both untouched: `K`/`J` and ctrl+u/ctrl+d.
- Anyone who prefers the old behaviour could restore it via config.
- Today the asymmetry runs the other. To get paging on PgUp/PgDn you must first discover the key is bound to an entirely different action in just one of the panels.

Still, there will definitely be a cost for existing users, no two ways about it. This will require an executive decision from the project, I can only make the case for what I want to see happen :)

### Describe alternatives you've considered

**Swap them in config.** Works today, and is what I'm running:

```yaml
keybinding:
universal:
prevPage:
nextPage:
scrollUpMain: ','
scrollDownMain: '.'
```

That's fine for me, but it doesn't address the default, which is the part that costs new users — and it only works because I already knew the key was bound to something else. That's the discovery step this issue is about.

### Additional context

- lazygit `v0.64.0`, git `2.55.0`, Linux.
- #1709 (open since 2022) implements the #1707 approach with `alt+f`/`alt+b`. It predates both the focusable main view and the modifier-key support that landed since, so I don't think it overlaps with this.

Start by locating the keybinding definitions for prevPage, nextPage, scrollUpMain, and scrollDownMain, then inspect how focused views receive these actions. Change the default PgUp/PgDn bindings as described while preserving the existing main-panel alternatives, and verify that paging follows the focused view and config overrides still work.

Written by the indexing model from the issue text.