feat: add ui.startFoldersOpenDepth config option

[nasxisbest]

Summary

• [] Closes issue #...
• I have read the CONTIBUTING.md and AI_POLICY.md guides

Adds ui.startFoldersOpenDepth config option to control how many levels deep folders are expanded when diffnav starts.

• -1 (default): all folders open, same as current behavior
• 0: all folders collapsed
• 1: first level open, everything deeper is collapsed
• N: N levels open

AI disclosure: Written with Claude Code. I reviewed all changes and understand the code. The closeDirsBelow function recursively walks children-first, then closes any directory node whose depth exceeds maxOpenDepth. Children-first traversal is needed because the charm bubbles tree hides a closed node's children from iteration via lipgloss Offset.

How Did You Test this Change?

• Unit tests for depth 0 (all collapsed) and depth 1 (first level open, deeper closed)
• go test ./... passes
• go build ./... clean

Images/Videos

[dlvhdr]

Hey @nasxisbest, thanks for the contribution :)
I understand why this can be useful but can you further explain what's your case for this?

[nasxisbest]

Yea, there's some project with different folder architectures. Monorepos, src / test folders, etc. etc.
This option allows them to tailor the diffnav to their use case better.

E.g. for monorepos - you could set the ui.startFoldersOpenDepth to 1 to only see the high-level project folders and see the diff for the whole folder.

[nasxisbest]

Implemented the suggestions, thank you for the review!

[nasxisbest]

Working on CI fixes.

[nasxisbest]

On a second thought, I do think it would be cool to have shortcuts for how deep the folders are opened. hhahaha

[dlvhdr]

thank you :)

[nasxisbest]

Thank you too!

[nasxisbest]

I'll file a follow up PR if I find the shortcuts useful.