feat: build desktop UI surfaces from atomic Livewire and Blade components

[ibourgeois]

Context

As a maintainer, I need the desktop UI to be built from small reusable components so Katra stays consistent, maintainable, and adaptable as the product shell evolves.

Acceptance Criteria

• Desktop UI work is required to use reusable Livewire or Blade components rather than ad hoc raw HTML fragments.
• The issue defines the expected component approach for app-shell primitives such as sidebars, headers, list rows, cards, badges, tabs, states, and panels.
• New MVP shell work reuses shared components instead of repeating markup for similar elements.
• The component approach supports feature-flagged or placeholder UI states without duplicating layout code.
• The issue documents the expectation that UI primitives should be small, composable, and suitable for reuse across future app surfaces.

Notes

This issue should reinforce maintainability and reuse while the product is still changing quickly.

The goal is to avoid baking one-off markup directly into pages that will later need to be reused across conversations, tasks, artifacts, agents, settings, and future forks of the Katra desktop foundation.

Out of Scope

• creating every possible component in a single PR
• adopting a different frontend framework solely for componentization