feat(desktop): align Codra UI with DP Code-style agent shell

[Abdulmuiz44]

Summary

• rework the desktop app shell into a desktop-first two-column layout with a fixed left sidebar and main workspace pane
• make dark mode the default and retune the dark theme to feel closer to DP Code, Linear, and Raycast
• tighten the sidebar, top bar, empty composer state, and floating model picker so the UI reads like a coding-agent desktop app instead of a stacked mobile layout

What changed

• app shell now uses a persistent sidebar + main content layout that stays side-by-side at desktop and narrow preview widths
• sidebar was simplified into DP Code-style navigation with Threads and Workspace sections, actions, grouped thread rows, and settings at the bottom
• main panel now has a compact top bar, centered empty composer, and slim browser-only Tauri warning banner
• model picker now opens as a floating nested popover near the composer with provider and model columns
• theme logic now defaults to dark when no saved preference exists and still persists manual toggles

Validation

• pnpm check:task-loop-protocol-sync
• pnpm --filter desktop build
• cargo check -p codra-core
• cargo test -p codra-core
• cargo check -p desktop

Notes

• browser preview was checked at roughly 1440px and 700px widths to confirm the sidebar stays left, the composer stays centered, and the layout no longer stacks like mobile on desktop-like widths
• there is an unrelated local modification in apps/desktop/src-tauri/gen/schemas/capabilities.json that was intentionally excluded from the commit and this PR