Brand kit JSON — one config file, every composer reads it

[manujajay]

Right now brand mark (`brandLead` + `brandLabel`), font choices, and footer URL are configured per-generation. For a team workflow, they should come from a single brand-kit file.

What this looks like

1. Define a `BrandKit` type in a new `src/lib/brand-kit.ts`:
   ```ts
   export type BrandKit = {
   name: string;
   brandLead: string;
   brandLabel: string;
   footerUrl: string;
   palette: { primary: string; secondary: string; accent: string };
   fonts: { headline: string; body: string };
   voice: { tone: string; bannedWords: string[] };
   };
   ```
2. Read `brand-kit.json` from the project root if present; otherwise use the built-in default.
3. Pipe brand-kit values into every composer template's defaults (currently hardcoded in `src/lib/composers.ts`).
4. Add a small "Brand kit" panel to the Studio that shows the active kit + a "Switch" dropdown if multiple JSON files exist.
5. Ship an example `brand-kit.example.json` in the repo root.

Why this matters

This is the single biggest "team" feature for the open-source build. Everyone who clones the repo is going to want to rip out our brand defaults and put theirs in.