fix: browser_tests Phase 1 - mechanical fixes

- Rename dragAndDrop to dragDrop (7 occurrences)

- Add override modifiers in SidebarTab.ts (4 fixes)

- Remove .ts import extensions in actionbar.spec.ts

- Prefix unused variables with underscore (9 files)

- Fix ESLint import() type annotation in globals.d.ts

Reduces typecheck:browser errors from 229 to 215

Amp-Thread-ID: https://ampcode.com/threads/T-019c1787-c781-761d-b95a-4844e909e64c
Co-authored-by: Amp <amp@ampcode.com>

browser_tests/fixtures/helpers/CommandHelper.ts

@@ -7,7 +7,7 @@ export class CommandHelper {
 
   async executeCommand(commandId: string): Promise<void> {
     await this.page.evaluate((id: string) => {
-      return window.app.extensionManager.command.execute(id)
+      return window.app!.extensionManager.command.execute(id)
     }, commandId)
   }
 
@@ -17,7 +17,7 @@ export class CommandHelper {
   ): Promise<void> {
     await this.page.evaluate(
       ({ commandId, commandStr }) => {
-        const app = window.app
+        const app = window.app!
         const randomSuffix = Math.random().toString(36).substring(2, 8)
         const extensionName = `TestExtension_${randomSuffix}`
 
@@ -41,7 +41,7 @@ export class CommandHelper {
   ): Promise<void> {
     await this.page.evaluate(
       ({ keyCombo, commandStr }) => {
-        const app = window.app
+        const app = window.app!
         const randomSuffix = Math.random().toString(36).substring(2, 8)
         const extensionName = `TestExtension_${randomSuffix}`
         const commandId = `TestCommand_${randomSuffix}`