Enhanced Finder with item_type hints and specialized picker support

[Aitai]

This PR implements the enhanced Finder architecture discussed in #1748, adding item_type hints that enable specialized picker providers while maintaining fallback compatibility.

Key changes:

• Added item_type support to Finder (branch, commit, tag, any_ref, stash, file)
• Created fzf-lua provider with specialized pickers (git_branches, git_commits, git_tags, git_status, etc.)
• Refactored diff popup with discoverable range actions (Branch Range, Commit Range, Tag Range) plus flexible Custom Range
• Auto-populates entries based on item_type when none provided
• Maintains intelligent fallback to generic pickers when specialized ones unavailable

This provides the hybrid UI approach discussed: common operations are discoverable while power-user scenarios (like diffing CHERRY_PICK_HEAD against a tag) are handled by the flexible Custom Range action using any_ref item_type.

Closes #1748