Fix Starship install failure: install to ~/.local/bin to avoid root escalation

[fan759]

Problem

setup_ubuntu.sh step 3/8 Starship fails to install:

! Escalated permissions are required to install to /usr/local/bin
> Installing Starship as root, please wait…
x Command failed (exit code 22): curl --fail --silent --location --output /tmp/....tar.gz \
    https://github.com/starship/starship/releases/latest/download/starship-x86_64-unknown-linux-musl.tar.gz

The upstream message ("This is likely due to Starship not yet supporting your configuration") is a red herring — x86_64-unknown-linux-musl is supported (current latest v1.25.1 ships that asset), and the tarball downloads fine as a normal user. That text is just the installer's generic fallback for any download failure.

Root cause

The upstream installer defaults its bin dir to /usr/local/bin, which isn't user-writable, so it re-runs the download under sudo (Installing Starship as root…). curl exit code 22 = --fail saw an HTTP status ≥ 400 during that root-context download. Common causes: sudo strips proxy env vars (env_reset), so a corporate proxy is bypassed and the request gets a 4xx; or a transient GitHub rate-limit. The identical download succeeds as the normal user.

Fix

Pass --bin-dir "$HOME/.local/bin" to the installer so the install stays entirely in user space and never escalates to root — eliminating the failing step. That directory is already created (for the bat symlink) and is first on PATH, so it matches the script's existing conventions.

macOS uses brew install starship, so it is unaffected.