vscode: add Logs sidebar panel for real-time debug visibility

[Allann]

Problem

When the extension fails to connect (e.g. clicking "AI Dev: Disconnected" and the backend silently fails to start), there's nowhere to see why. All logs go to the Output Channel — a separate panel users rarely open — and backend stdout/stderr is piped but never read.

Changes

New: Logger class (src/Logger.ts)

Replaces the raw vscode.OutputChannel with a structured logger that:

• Wraps the Output Channel (still writes there)
• Maintains a circular buffer of 1000 LogEntry objects ({ timestamp, level, message })
• Exposes info(), warn(), error(), and appendLine() (backward-compat alias — zero call-site changes)
• Subscriber pattern (subscribe() / onCleared()) for streaming to the webview
• getHistory() for sending buffered entries when the view first opens

New: LogsPanelProvider (src/panels/LogsPanelProvider.ts)

A WebviewViewProvider that:

• Subscribes to Logger and streams entries to the webview in real time
• Sends full history when the view opens or becomes visible
• Handles { type: 'clear' } from the webview → calls logger.clearBuffer()
• Always live — doesn't wait for a backend connection

New: Logs webview (src/webviews/logs/main.tsx)

React-based log viewer with:

• Color-coded rows: default = info, yellow = warn, red = error
• HH:MM:SS LEVEL message format
• Auto-scroll to latest entry (pauses when user scrolls up)
• Clear button

Extended: BackendProcessManager

• Added stdout?: NodeJS.ReadableStream | null and stderr?: NodeJS.ReadableStream | null to ProcessLike
• Added onOutput?: (line: string, source: 'stdout' | 'stderr') => void option
• Uses readline.createInterface to capture process output line-by-line (previously silently discarded despite stdio: 'pipe')
• Existing test mocks are unaffected (new fields are optional)

Updated: extension.ts

• Swaps vscode.OutputChannel → Logger
• Registers aidev.logs webview view
• Passes onOutput callback to BackendProcessManager (stderr → log.warn, stdout → log.info)

Config: package.json + esbuild.mjs

• Registers aidev.logs as a fourth webview view in the ai-dev-studio activity bar container
• Adds logs/main.tsx to the esbuild webview entry points

Testing

• All 29 existing unit tests pass
• Build succeeds (npm run build)