Feature: Multiple grpc targets

[pranavpsawant]

Summary

Adds first-class multi-target support — a single gRPC Studio instance can now
connect to several gRPC servers at once and switch between them from a server
selector in the UI. Also ships a second bundled example service, restructures the
examples, and adds demo/deployment tooling. This is a breaking change to the
backend and Helm config (single target → list of targets), so the app and chart
are bumped to 2.0.0.

Features

Multi-target

• Backend connects to multiple gRPC targets defined under client.targets[]
  (each with its own host/port/mode and TLS/mTLS settings); at least one is
  required. Per-target client management via a new multi-client manager.
• Server selector in the UI to choose which target(s) to explore.
• Method tabs — open many methods side by side, duplicate and close tabs.
• Indexed env overrides (GRPC_TARGET_<i>_HOST/PORT/...) for per-target config.

Second example service

• New BookStore example gRPC server (:50052) mirroring PetStore (:50051),
  exercising the full proto3 feature set (nesting, oneOf, maps, wrappers, Struct,
  Any, FieldMask, all four RPC modes).
• Examples restructured example/ → examples/{petstore,bookstore}/.
• Default config now targets both, so they appear side by side out of the box.

Developer experience

• Rewritten Quick Start in the README: install → try-it-all → run pieces
  individually → point at your own server.
• New scripts: dev:all (UI + backend + both examples), dev:petstore,
  dev:bookstore, dev:multi, and stop (frees dev ports; auto-runs before
  every dev* command to prevent EADDRINUSE).
• quickstart now accepts multiple targets (-- 50051 50052, named
  --target name=host:port,mode=tls) and generates a temp config.

Demo assets

• Programmatic capture via Playwright: demo:capture (screenshots + a chaptered
  walkthrough video) and demo:gif / demo:mp4 converters.
• Walkthrough covers multi-server, search, tabs, Form/JSON/Schema, all streaming
  modes, request history, and shareable URLs.

Deployment

• Helm chart migrated to connection.targets[] (removed dead
  connection.timeout.connect/tls.* keys); shared mTLS secret across mtls
  targets; updated examples, NOTES, and docs.
• CI (docker-publish.yml) now also builds & publishes multi-arch petstore
  and bookstore images to GHCR (fixed out-of-sync example lockfiles).

Layout fix

• App fits a single page in its empty state and scrolls the outer page (not an
  inner panel) when content is tall; Service/Method explorer columns are equal height.

Breaking changes

• Backend client.mode/client.target → client.targets[] (min 1, each needs a name).
• Helm connection.target/mode → connection.targets[].
• See MIGRATION.md and helm/MIGRATION.md.

Testing

• Backend: 365/365 pass. Frontend: 511/511 pass. helm lint clean;
  all example configs validate against the schema; both example Docker images
  build and start (reflection enabled). Scanned clean for secrets/IDs.