feat: (optional) arrow folder expansion in the file tree

[N4M3Z]

The default for h / l is unchanged: they stay as horizontal scroll, matching vim's character navigation. Users who want gitui-style tree nav opt in via config.

• New config arrow_tree_navigation (default false). When set to true, h / l in the file list fall through to gitui-style tree nav at the horizontal scroll boundary: l expands a collapsed folder, descends into an expanded one, or jumps to the next folder below on a file; h collapses an expanded folder, otherwise ascends to the parent (and at the top level jumps to the previous folder above).
• With the flag on, moving the file-list cursor (j / k, arrows, or the tree-nav keys above) to a file also scrolls the diff to that file's header without changing focus. Folders are a no-op so arrowing past collapsed entries leaves the diff alone. Matches gitui / lazygit preview behavior. Enter still commits the selection and shifts focus.
• Horizontal scroll keeps the sticky prefix (indent, expand icon, checkbox, status badge) anchored at the left edge; only the filename portion slides. Scroll caps at "at least one column visible" so a name never disappears entirely. Leaving the file list resets scroll_x to 0.
• Enter / Space still toggle folders directly, regardless of the flag.
• ADR: docs/decisions/FEAT-0014 Folder Expansion.md

Test plan

• cargo fmt && cargo clippy -- -D warnings clean
• Manual: default config keeps h/l as plain horizontal scroll
• Manual: with arrow_tree_navigation = true, l on a folder expands; second l descends; l on a file jumps to the next folder
• Manual: with arrow_tree_navigation = true, h on an expanded folder collapses; on a file jumps to the containing folder; at top level jumps to the previous folder
• Manual: with arrow_tree_navigation = true, arrowing the file-list cursor across files scrolls the diff to the new file's header; arrowing over folders does not move the diff; focus stays on the file list
• Manual: with arrow_tree_navigation = false, arrowing the file-list cursor leaves the diff viewport alone (Enter still required to jump)
• Manual: horizontal scroll on a long filename keeps the checkbox and status badge visible at the left edge; only the name slides
• Manual: switching focus away from the file list and back returns long names to their starting position

[N4M3Z]

Last larger feature I have in the pipeline, I'll send something cosmetic and that's gonna be it for a while. We should split app.rs into several modules so that we can also split off tests by concern, I might get to it in a couple week

[agavra]

Thanks @N4M3Z let's split this into different PRs. I like the mouse down change but I don't want to repurpose h and l for something other than scrolling, this is primarily meant to "feel" like vim, not gitui

also I'll break up app.rs soon, that's on my todo list - i totally agree we need that

[N4M3Z]

Sure, I'll split it, but please give the folder expansion a shot, it makes it super quick to navigate the folder tree.

[N4M3Z]

Minimized the feature set, made the behaviour optional