Enable resizable, scrollable shadcn sidebars

[deadronos]

Motivation

• Improve the new shadcn-inspired app shell so left/right sidebars can be reopened, resized and can scroll independently when their nested contents overflow.
• Address the UX feedback that sidebars could not be reopened after collapse and long panel content was not scrollable.

Description

• Added left/right collapse state, width state, draggable resize handles, and mouse event handlers to src/App.tsx, and applied those widths via CSS variables to the shell grid layout.
• Made sidebar groups and nested SidebarSection contents independently scrollable and added resizer elements with accessible roles and hover affordances in src/styles.css.
• Added a reusable SidebarSection component at src/ui/SidebarSection.tsx for collapsible nested sections and applied shadcn-like surface tokens and responsive rules to src/styles.css.
• Updated memory/task docs (memory/*) to record the resizable/scrollable sidebar work and fixed small TS typing imports (CSSProperties / ReactMouseEvent) to satisfy the new inline style and handler types.

Testing

• Ran pnpm typecheck, which failed because the generated WASM bridge module src/gen/rust_engine is not present in this environment, producing import resolution errors.
• Ran pnpm lint, which surfaced preexisting wasmLoader unsafe-assignment warnings (these are unrelated to the sidebar changes and remain to be addressed separately).
• Ran pnpm test; the test run executed the suite (many unit tests passed), but multiple wasm-related suites failed due to the missing src/gen/rust_engine artifact and Vite import resolution; attempted UI screenshot capture also failed because the WASM/browser environment could not be fully exercised.

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Codex Task