feat: Support custom keybindings via config.toml

[shatrughantwt]

Fixes: #45

Summary

This PR adds support for user-configurable keybindings via config.toml with safe fallback to existing defaults.

Previously, all keybindings were hardcoded in internal/tui/keys.go.
Now users can override specific actions in [keybindings] without modifying source code.

What Changed

• Added [keybindings] support in config parsing:
  • Extended config struct to include Keybindings map[string]string.
• Added keybinding merge logic:
  • New KeyMapFromConfig(...) applies user overrides on top of DefaultKeyMap().
  • If a keybinding is missing, default is used.
  • Empty/invalid values are ignored and default remains.
  • Unknown actions are ignored (logged).
• Wired keybinding resolution at startup:
  • initialModel() now loads config and initializes global keys from merged config + defaults.
• Updated default generated config file:
  • Includes:

    theme = "GitHub Dark"
    
    [keybindings]

Example

theme = "GitHub Dark"

[keybindings]
quit = "x,ctrl+c"
up = "w,up"
down = "s,down"
toggle_help = "h"
open = "enter"

Notes

• open is supported as an alias for checkout.
• Keybindings are loaded on startup (restart required after config edits).

Tests

• Added tests for:
  • override behavior
  • fallback behavior
  • multi-key parsing ("x,ctrl+c")
  • invalid/empty override fallback

Manual Verification

• Verified remap works (quit = "x"):
  • q no longer quits
  • x quits
• Verified fallback works when override is removed:
  • default q quits again.