docs: add setup command and refresh README

Add a project-level setup recipe for the initial development bootstrap. The
recipe installs the Tauri CLI and resolves the Tauri frontend dependencies from
the Deno lockfile so new contributors have one command before running or
building the app.

Refresh the README to keep it short and focused on what Lanspread is, how to
bootstrap it, and the most important just recipes.

Test Plan:
- just --summary
- just --dry-run setup
- git diff --check
- git diff --cached --check

justfile

@@ -2,6 +2,10 @@ export RUSTFLAGS := "-C target-cpu=native"
 export WEBKIT_DISABLE_COMPOSITING_MODE := "1"
 export DOCKER_CONFIG := env_var_or_default("DOCKER_CONFIG", ".lanspread-peer-cli/docker-config")
 
+setup:
+    cargo install tauri-cli
+    cd crates/lanspread-tauri-deno-ts && deno install --frozen=true
+
 run:
     cargo tauri dev --release