feat: settings icon and keyboard shortcuts section in sidebar

[mishraprafful]

Closes #37
Closes #35

Summary

• Adds a gear icon button at the bottom-right of the sidebar that opens the Settings panel (same as ⌘,)
• Adds a keyboard icon button at the bottom-left that toggles a collapsible shortcuts reference panel
• Shortcuts panel lists all supported bindings: ⌘N, ⌘,, ⌘B, ⌘1-9, Esc×2

Test plan

• Click gear icon → Settings panel opens
• Click gear icon again (or ⌘,) → Settings panel closes
• Click keyboard icon → shortcuts panel expands above the footer bar
• Click keyboard icon again → panel collapses
• All 5 shortcuts are listed correctly

Screenshots

🤖 Generated with Claude Code

[greptile-apps]

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