feat(storybook): add Storybook for nodes and shared components

[Horuse]

What does this PR do?

Adds Storybook and replaces the /preview route.

• Stories for all 27 nodes plus Nodes/Gallery. Each node runs in a real flow canvas with controls for node data, wired channels, a fake signal and mocked engine state (devices, running, platform, plugins, network, WebRTC).
• Stories for shared form controls and components (Combobox, Toggle, Slider, Number Stepper, Segmented Buttons, Password Input, Meter Bar, Signal Bars, Copy Button).

Why is this the right approach?

/preview showed one fixed state per node without handles. Storybook gives per-state stories and live controls with no Storybook-only code in the app components: all mocks sit in src/stories/harness/ (mockIPC + signal generator).

New dev deps: storybook, @storybook/sveltekit, @storybook/svelte, @storybook/addon-svelte-csf. .storybook/main.ts keeps Svelte-source deps out of esbuild pre-bundling. This works around the existing vite 6 / vite-plugin-svelte 7 mismatch.

Checklist

• Diff is limited to the change — no unrelated edits
• bun run check passes
• cargo check — N/A, no Rust changes
• bun run format leaves the tree clean
• Generated TS types — N/A
• No new dependency without a reason in the PR description
• No changes to the RT audio path

Platform coverage

• Developed on: macOS
• Tested on: macOS
• What I did to test: bun run check, manual check of node stories, smoke pass over 191 of 230 stories with no errors
• Not tested: Linux / Windows. Frontend tooling only, no per-OS files touched.