UI refresh: toolbar icons, tooltips, staging fixes, theme rename

[tspython]

Summary

• Refreshed toolbar with bundled SVG icons, distinct plus/plus-circle and minus/minus-circle glyphs so single-file vs all-file stage/unstage actions read at a glance.
• Added hover tooltips on toolbar buttons (500 ms delay, dark chip with shortcut, drop shadow, dedicated palette slot so contrast holds on every theme).
• Fixed two file-list bugs:
  • Mouse clicks and j/k selected the wrong file when sections reordered alphabetical git status output (visual index ≠ entries Vec index).
  • Files with split state like MM / AM / MD only appeared in one section; they now show up in every section they belong to.
• Renamed the default theme Warp Dark → Midnight and removed every warp reference from identifiers, comments, and docs.
• Added /build and .DS_Store to .gitignore.

Test plan

• Hover toolbar icons — tooltip appears after ~500 ms, stays readable on every bundled theme
• Click any file in the file list — s / u / x operate on the file you actually clicked
• Modify a staged file in the working tree — it appears in both Staged and Unstaged sections
• WGIT_THEME=midnight / gruvbox / vercel / dracula all load
• cargo test passes (17 unit + 13 integration)

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