feat: add external terminal support with cross-platform detection

Add support for opening worktree directories in external terminals
(iTerm2, Warp, Ghostty, System Terminal, etc.) while retaining the
integrated terminal as the default option.

Changes:
- Add terminal detection for macOS, Windows, and Linux
- Add "Open in Terminal" split-button in worktree dropdown
- Add external terminal selection in Settings > Terminal
- Add default open mode setting (new tab vs split)
- Display branch name in terminal panel header
- Support 20+ terminals across platforms

Part of #558, Closes #550

package-lock.json

@@ -11275,7 +11275,6 @@
       "os": [
         "darwin"
       ],
-      "peer": true,
       "engines": {
         "node": ">= 12.0.0"
       },
@@ -11297,7 +11296,6 @@
       "os": [
         "darwin"
       ],
-      "peer": true,
       "engines": {
         "node": ">= 12.0.0"
       },
@@ -11341,7 +11339,6 @@
       "os": [
         "linux"
       ],
-      "peer": true,
       "engines": {
         "node": ">= 12.0.0"
       },
@@ -11363,7 +11360,6 @@
       "os": [
         "linux"
       ],
-      "peer": true,
       "engines": {
         "node": ">= 12.0.0"
       },
@@ -11385,7 +11381,6 @@
       "os": [
         "linux"
       ],
-      "peer": true,
       "engines": {
         "node": ">= 12.0.0"
       },
@@ -11407,7 +11402,6 @@
       "os": [
         "linux"
       ],
-      "peer": true,
       "engines": {
         "node": ">= 12.0.0"
       },
@@ -11429,7 +11423,6 @@
       "os": [
         "linux"
       ],
-      "peer": true,
       "engines": {
         "node": ">= 12.0.0"
       },
@@ -11451,7 +11444,6 @@
       "os": [
         "win32"
       ],
-      "peer": true,
       "engines": {
         "node": ">= 12.0.0"
       },
@@ -11473,7 +11465,6 @@
       "os": [
         "win32"
       ],
-      "peer": true,
       "engines": {
         "node": ">= 12.0.0"
       },