feat(shellv2): add ssh meta function and tabbed interface

[KCarretto]

This PR introduces a new "meta functions" feature to the ShellV2 interface, starting with the ssh("user:password@host") command.

Changes include:

• Rust WASM Parser: Modified eldritch-wasm's BrowserRepl to intercept the ssh command and return a special JSON payload ({"status": "meta", ...}) instead of sending the command to the agent.
• Tabbed UI: Refactored the ShellV2 component to use Chakra UI's Tabs, allowing the main shell and newly spawned SSH sessions to exist side-by-side.
• SSH Terminal Component: Created a new SshTerminal component that uses xterm.js and connects to the backend via the /portals/ssh/ws WebSocket endpoint. It handles session UUID generation, sequence ID tracking (incrementing only on sends), and automatic reconnections.
• State Management: Fixed a stale closure bug by using a useRef for portalId within useShellTerminal to ensure the handleMetaSsh callback always has access to the current portal state.
• Rendering Fixes: Added a synthetic window resize event when switching tabs to ensure xterm.js's FitAddon correctly calculates dimensions for terminals that were previously hidden.

The SSH functionality requires an active portal connection to function; if invoked without one, an error message is printed directly to the shell terminal.

Testing performed:

• Automated frontend verification using Playwright.
• Backend tests passing (go test ./tavern/internal/portals/ws/... -v).
• Rust tests passing (cargo check and cargo test).

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PR created automatically by Jules for task 4121754529262080486 started by @KCarretto

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