ISSUE-738: Refactor Left Panel

[ada333]

This PR refactors LeftPanel into functional component using custom hooks. This make the component much more simple - from 800 lines to 300. The 6 hooks introduced in here ->

Hook	Responsibility
useNotebookMetadata	Owns all pipeline metadata state (experiment, pipeline name/description, base image, caching, namespace, etc.) and exposes granular updaters for each form field. Composes the three hooks below.
useNotebookLoader	Subscribes to the notebook tracker's currentChanged signal and runs the async loading sequence (wait for session, fetch base image/namespace/experiments via RPC, read .ipynb metadata, merge defaults).
useNotebookMetadataPersistence	Writes metadata back to the .ipynb file whenever the state changes.
useEnableByDefaultEffect	Reacts to the "Enable Kale by default" JupyterLab setting: when it transitions from off→on, it flips the toggle without requiring a notebook switch.
useKfpStatus	Polls kfp.ping every 30s and returns the connection status for the header badge.
useDeployment	Manages the full compile/upload/run lifecycle: triggering deploys, tracking progress per deploy index, handling unsaved-changes prompts, and exposing callbacks for the toolbar and deploy button.

Other important changes:

1. Types extracted to LeftPanelTypes.ts
   DeployType, IExperiment, IKaleNotebookMetadata, DefaultState, and NEW_EXPERIMENT moved to a dedicated types file. LeftPanel.tsx re-exports them so existing imports from other files continue to work without changes.

2. Toolbar decoupled from the class instance
   kaleToolbar.ts no longer imports KubeflowKaleLeftPanel directly. Instead it defines an ILeftPanelHandle interface (triggerCompile, triggerRun, isKaleEnabled) and exposes setLeftPanelCallbacks. The function component registers its callbacks via a useEffect, eliminating the React ref that was previously threaded through widget.tsx.

3. Utility functions made standalone
   getNotebookFileName, sanitizePipelineName, and getNotebookPath were extracted from class methods into plain functions in useNotebookLoader.ts — they had no dependency on this and are now easier to unit-test.

I tested this by simply running JupyterLab and trying Left Panel in different ways. But this definitely needs thorough testing - there are lot of different scenarios.

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[ederign]

@ada333 keeps those PR's coming, I'm just waiting to branch for 2.0 to start to review and merge those.