feat: add open_in_prev_tab keymap and fix keymap leak on tab switch

[esmuellert]

Summary

Adds a new keymap and fixes a keymap leak issue when switching between tabs.

Changes

New Feature: open_in_prev_tab keymap (gf)

• Opens the current diff buffer in the previous tab (or creates a new tab before current if none exists)
• Only works on real file buffers (not virtual/git revision buffers)
• Preserves cursor position when switching

Bug Fix: Keymap leak on tab switch

• Previously, keymaps set on diff buffers would persist when switching to other tabs, causing conflicts
• Now clears keymaps on TabLeave and reapplies them on TabEnter
• Uses a reapply_keymaps callback pattern to avoid stale closures referencing old buffer IDs
• Refactored cleanup logic to use centralized clear_tab_keymaps accessor

Testing

• Verified keymaps are cleared when leaving diff tab
• Verified keymaps are restored when returning to diff tab
• Tested gf keymap opens buffer in previous tab correctly

Closes #67

[chapeupreto]

Thank you very much!! 👏