feat(ui): migrate app shell to Tailwind CSS + shadcn (dark theme)

[Shakalito]

Introduce Tailwind CSS and shadcn UI as the primary styling system.
Establish a dark-theme baseline inspired by trading platforms.

AC:

• Tailwind configured and used as main styling solution

• shadcn components for buttons, inputs, cards, tables

• Dark theme baseline applied consistently

• At minimum: Landing, Dashboard, Portfolio, Alerts styled

Deps - none.

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To ensure a smooth migration and maintain code quality, I propose breaking this task into 4 sequential Pull Requests (PRs).
This approach allows for better testing and prevents massive, hard-to-review code changes

• a: chore(ui): add Tailwind CSS base setup

  • Scope: Installation, tailwind.config.js, PostCSS, and base global styles.

  • AC: Tailwind utility classes work on a test page; no regressions in existing legacy CSS.

• b: chore(ui): add shadcn/ui + base theme tokens

  • Scope: shadcn init, installation of core components (Button, Input, Card, Table). Setup of CSS variables for Dark Mode.

  • AC: Ability to use shadcn components in a single view; theme tokens correctly applied.

• c: feat(ui): create AppShell layout (nav + container)

  • Scope: Building a unified wrapper for authenticated routes (Navbar + Sidebar/Container).

  • AC: Portfolio, Alerts, and Watchlist pages use the same master layout.

• d: feat(ui): apply dark theme to core pages

  • Scope: Full style migration for Dashboard, Portfolio, Alerts, and Landing Page. Removal of legacy inline styles and bare HTML tables.

  • AC: All core pages are responsive and follow the XTB-inspired Dark Theme.