Default Studio host to 127.0.0.1 and prompt before auto-start (#5267)

Studio bound to 0.0.0.0 by default and the installer silently auto-started
a server at end of install, exposing it on the network without consent and
contradicting the privacy-first / local-only guarantee.

- studio/backend/run.py: run_server() and argparse --host default to 127.0.0.1
- unsloth_cli/commands/studio.py: studio_default() and run() --host default to 127.0.0.1
- install.sh: drop -H 0.0.0.0 from generated launcher template; replace silent
  auto-start with a [Y/n] prompt; add cloud/network note to manual hint
- install.ps1: drop -H 0.0.0.0 from PowerShell launcher template; replace
  silent auto-start with a Read-Host [Y/n] prompt; add cloud/network note
- studio/setup.sh: drop -H 0.0.0.0 from launch hint; add cloud/network note
- README.md: simplify launch examples to `unsloth studio -p 8888`; note
  -H 0.0.0.0 is available for cloud/LAN use

Tests:
- studio/backend/tests/test_host_defaults.py
- tests/studio/test_cli_studio_defaults.py
- tests/sh/test_install_host_defaults.sh

README.md

@@ -79,8 +79,9 @@ irm https://unsloth.ai/install.ps1 | iex
 
 #### Launch
 ```bash
-unsloth studio -H 0.0.0.0 -p 8888
+unsloth studio -p 8888
 ```
+> For cloud VMs or LAN access, add `-H 0.0.0.0` to bind on all interfaces.
 
 #### Update
 To update, use the same install commands as above. Or run (does not work on Windows):
@@ -167,7 +168,7 @@ The below advanced instructions are for Unsloth Studio. For Unsloth Core advance
 git clone https://github.com/unslothai/unsloth
 cd unsloth
 ./install.sh --local
-unsloth studio -H 0.0.0.0 -p 8888
+unsloth studio -p 8888
 ```
 Then to update :
 ```bash
@@ -180,7 +181,7 @@ git clone https://github.com/unslothai/unsloth.git
 cd unsloth
 Set-ExecutionPolicy -Scope Process -ExecutionPolicy Bypass
 .\install.ps1 --local
-unsloth studio -H 0.0.0.0 -p 8888
+unsloth studio -p 8888
 ```
 Then to update :
 ```bash
@@ -193,11 +194,11 @@ git clone https://github.com/unslothai/unsloth
 cd unsloth
 git checkout nightly
 ./install.sh --local
-unsloth studio -H 0.0.0.0 -p 8888
+unsloth studio -p 8888
 ```
 Then to launch every time:
 ```bash
-unsloth studio -H 0.0.0.0 -p 8888
+unsloth studio -p 8888
 ```
 
 #### Nightly: Windows:
@@ -208,11 +209,11 @@ cd unsloth
 git checkout nightly
 Set-ExecutionPolicy -Scope Process -ExecutionPolicy Bypass
 .\install.ps1 --local
-unsloth studio -H 0.0.0.0 -p 8888
+unsloth studio -p 8888
 ```
 Then to launch every time:
 ```bash
-unsloth studio -H 0.0.0.0 -p 8888
+unsloth studio -p 8888
 ```
 
 #### Uninstall