feat(filepicker): add search/filter functionality

[toller892]

Summary

Adds incremental search to the file picker component, allowing users to filter visible files by name within the current directory.

Motivation

Finding a specific file in a large directory is tedious — you have to scroll through everything. This adds vim-style / search to filter files in place. (bubbletea#1685)

Usage

Key	Action
/	Enter search mode
Type	Filter files (case-insensitive substring match)
Enter	Confirm filter, return to browsing
Esc (in search)	Exit search mode; if filter was applied, clear it
Esc (browsing)	Clear active filter; if no filter, navigate to parent dir

All navigation keys (j/k/g/G/PgUp/PgDn) work on the filtered list. Entering a directory clears the search.

Implementation

• FilterFunc type + DefaultFilter (case-insensitive substring) for customizable filtering
• Filter field on Model — users can swap in fuzzy matching (sahilm/fuzzy) or any custom logic
• Search uses textinput.Model (same pattern as list component's filter)
• filteredFiles / filteredMap track the mapping between filtered and original indices
• Navigating into a directory or pressing Back clears the search automatically

Testing

go build ./filepicker/
go vet ./filepicker/
go test ./...

All pass. The filepicker has no existing test file; I can add tests if desired.