fix(examples): update installation instructions + cors origin url

[divyagnan]

Related: #1140

Why

Tried to run the examples from a fresh git clone using the existing example instructions and wasn't able to.

1. Running npm install gave the following error:

npm error code EUNSUPPORTEDPROTOCOL
npm error Unsupported URL Type "workspace:": workspace:*

2. One of the example readmes actually had pnpm install so I tried that and it worked.
3. However then running the dev command still failed due to not being able to find the @rivetkit/react package:

 npm run dev                                                                                                            

> example-game@0.9.8 dev
> concurrently "tsx --watch src/backend/server.ts" "vite"

[0]
[0] node:internal/modules/run_main:122
[0]     triggerUncaughtException(
[0]     ^
[0] Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/Users/dk/Code/experiments/rivetkit/examples/game/node_modules/@rivetkit/actor/dist/mod.js' imported from /Users/dk/Code/experiments/rivetkit/examples/game/src/backend/registry.ts
[0]     at finalizeResolution (node:internal/modules/esm/resolve:257:11)
[0]     at moduleResolve (node:internal/modules/esm/resolve:913:10)
[0]     at defaultResolve (node:internal/modules/esm/resolve:1037:11)
[0]     at nextResolve (node:internal/modules/esm/hooks:748:28)
[0]     at resolveBase (file:///Users/dk/Code/experiments/rivetkit/node_modules/.pnpm/tsx@4.20.3/node_modules/tsx/dist/esm/index.mjs?1753903454156:2:3811)
[0]     at resolveDirectory (file:///Users/dk/Code/experiments/rivetkit/node_modules/.pnpm/tsx@4.20.3/node_modules/tsx/dist/esm/index.mjs?1753903454156:2:4310)
[0]     at resolveTsPaths (file:///Users/dk/Code/experiments/rivetkit/node_modules/.pnpm/tsx@4.20.3/node_modules/tsx/dist/esm/index.mjs?1753903454156:2:5051)
[0]     at resolve (file:///Users/dk/Code/experiments/rivetkit/node_modules/.pnpm/tsx@4.20.3/node_modules/tsx/dist/esm/index.mjs?1753903454156:2:5428)
[0]     at nextResolve (node:internal/modules/esm/hooks:748:28)
[0]     at Hooks.resolve (node:internal/modules/esm/hooks:240:30) {
[0]   code: 'ERR_MODULE_NOT_FOUND',
[0]   url: 'file:///Users/dk/Code/experiments/rivetkit/examples/game/node_modules/@rivetkit/actor/dist/mod.js'
[0] }
[0]
[0] Node.js v22.11.0
[0] Failed running 'src/backend/server.ts'
[1]
[1]   VITE v5.4.19  ready in 640 ms
[1]
[1]   ➜  Local:   http://localhost:3000/
[1]   ➜  Network: use --host to expose
[1] Error:   Failed to scan for dependencies from entries:
[1]   /Users/dk/Code/experiments/rivetkit/examples/game/src/frontend/index.html
[1]
[1]   ✘ [ERROR] Failed to resolve entry for package "@rivetkit/react". The package may have incorrect main/module/exports specified in its package.json. [plugin vite:dep-scan]

4. I was able to fix this by running pnpm run build from the root directory of the repo.
5. After getting everything installed and running some of the examples still didn't work — I was getting this cors error (in the browser):

Access to fetch at 'http://localhost:8080/registry/actors/resolve' from origin 'http://localhost:3000' has been blocked by CORS policy: The 'Access-Control-Allow-Origin' header has a value 'http://localhost:5173' that is not equal to the supplied origin. Have the server send the header with a valid value.

6. Saw that some of the examples have an explicitly defined vite config to run the server at localhost:3000 (rather than the default which is localhost:5173). Updating the cors origin url in the server.ts files for these examples fixed the issue and made them work.

What does this PR do

1. Updates the example README.md files (for all examples) with the instructions that worked
2. Update the examples with the vite server url and cors origin url mismatches to match

Put them in separate commits bc you could also update the vite.config.ts server port url to fix the issue instead of updating the cors origin url in server.ts. Wasn't sure which would be preferred — seemed like the newer examples use the explicit port 3000 so decided to update the cors url.

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