feat(ui): show token counts per file & directory in tree

[WhiteHades]

Closes #532

Adds token counts next to each file and directory in the directory structure view so users can easily identify and exclude large items.

Changes:

• Added token_count field to FileSystemNode
• Token counting happens during content gathering
• Directory counts are bold, file counts are normal weight
• Summary shows total tokens (tree + content), matching root directory count

Testing:
Tested locally with various repo sizes - speed is slower than the current iteration given that it now counts the token per file.

[github-actions]

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