fix: updated docs to include API proxying walkthrough, theme system UI Consistency and dev fixes

[Vinyl-Davyl]

Description of the Change

• Documentation related to the change has been updated

This PR is a multi-faceted refinement of the Metaflow UI codebase, focusing on design system consistency, test reliability, and technical documentation for onboarding.

1. Documentation Improvements (README.md)

Detailed the Technical Infrastructure (API Proxying) to help new developers understand how the UI communicates with the backend via src/setupProxy.js

2. UI & Design System Consistency (theme.css)

• Fixed a critical typo in CSS variable naming (font-familty -> font-family).
  Corrected malformed CSS variable references where triple-dashes were used (---color-text-primary -> --color-text-primary).
• Fixed a bug in TaskListLabel.tsx where a link color used a bare string instead of a var() wrapper, causing regression in link visibility.

3. Internal Reliability & Quality

Code Quality: Performed a sweep for common typos in internal helpers

Alternate Designs

N/A

Possible Drawbacks

None. These are primarily bug fixes and documentation enhancements.

Verification Process

Ran the local development server and verified that the theme.css font overrides are now correctly applied to the Collapsable headers.
Verified that the Task List links are picking up the correct theme colors via the fixed var() call in TaskListLabel.tsx

Release Notes

Fixed several CSS variables and styling typos to ensure theme consistency across all UI components.
Improved the main README with architectural and technical details to assist new contributors.

[Vinyl-Davyl]

cc: @saikonen @romain-intel