Feature Request: File icon themes in the file explorer (Material Icon Theme support)

## Problem
The file explorer currently uses a single generic icon set. When working across
many worktrees and repos, visually scanning the tree is slower than in VS Code,
where folder/file icons (e.g. Material Icon Theme) let you identify `.claude/`,
`docs/`, `server/`, `build/` etc. at a glance.

## Proposal
Support file icon themes in the file explorer:
- Per-folder-name and per-extension icon mapping (like VS Code's file icon theme JSON)
- Ideally compatible with the VS Code icon theme format so existing themes
(Material Icon Theme, Seti, etc.) can be reused
- A minimal built-in themed set would already be a big win

## Why it fits Orca
- Orca already embeds Monaco (VS Code's editor), so the icon-theme JSON format
is a natural fit
- Repo icons are already customizable in the sidebar — this extends the same
idea into the file tree
- Heavy parallel-worktree users navigate the explorer constantly; icon themes
reduce scan time

## Alternatives considered
- Repo icons (sidebar only, doesn't help inside the tree)
- No plugin system exists today, so this can't be solved by an extension