fix: explain that Amazon Q is unavailable in browser-only VS Code

[laileni-aws]

Problem

When the Amazon Q extension is installed in the browser-only version of VS Code (for example vscode.dev or github.dev opened without a remote environment such as a codespace), the extension appears to be installed and shows a "Sign in" entry, but clicking it does nothing. On activation users also see an error notification:

Failed to launch Amazon Q language server: g.spawn is not a function

and the log contains:

[warning] amazonqLsp: Failed to start downloaded LSP, falling back to bundled LSP: Unable to find a language server that satifies one or more of these conditions: version in range [>=1.0.0-0 <2.0.0-0], matching system's architecture and platform
[error] focusAmazonQPanel failed: Error: command 'aws.amazonq.focusChat' not found

Amazon Q depends on the Amazon Q language server, which runs as a Node.js child process. The browser-only extension host cannot spawn processes, so Amazon Q cannot work there. Today nothing tells the user this; the extension just looks broken.

Solution

• On web activation (extensionWeb.ts), show a one-time informational notification explaining that Amazon Q is not available in the browser-only version of VS Code and that they should use desktop VS Code or connect the browser editor to a remote environment.
• Skip language server activation in web mode instead of attempting to spawn it and surfacing the confusing "Failed to launch Amazon Q language server" error.
• Skip the first-use focusAmazonQPanel call in web mode, since the chat view is intentionally hidden there (!aws.isWebExtHost) and the command is never registered.
• The status bar "Sign in to get started" entry in web mode now shows the same explanation instead of kicking off an auth flow that never leads anywhere.

New helper: packages/core/src/amazonq/webMode.ts (showWebModeUnsupportedMessage, showWebModeUnsupportedMessageOnce).

Testing

• Added packages/core/src/test/amazonq/webMode.test.ts (3 tests): the notification is shown as info, the web-mode sign-in entry shows it, and it is not shown outside web mode. All pass.
• Existing codewhisperer/commands/basicCommands.test.ts (40 tests, exercises createSignIn) passes.
• npm run testWeb -w packages/amazonq (46 tests) passes; the web extension host log now shows amazonqLsp: skipping language server activation: not supported in the web extension host and no longer reports the language server launch failure.
• npm run compileDev -w packages/amazonq builds both the Node and web bundles; eslint/prettier clean on changed files.

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