Rootfs: Restore the bash login shell so demo tab-completion works

[brandonpayton]

Why

Interactive tab auto-completion in the browser demo shell stopped working.
Bash provides that completion through readline; dash does not. PR #1307
("Boot login from package-backed images") changed the main-shell demo boot
from launching bash directly to booting through login:

before: argv = ["bash", "-l", "-i"] # interactive bash
after: argv = ["/usr/bin/login", "-p", "-f", "maker"]

login -f maker execs the maker account's login shell from
images/rootfs/etc/passwd, which was /bin/sh -- and /bin/sh is a dash
alias (see images/rootfs/PACKAGES.toml). So the demo silently went from an
interactive bash shell (with completion) to dash (with none).

What changed

Set the maker account's login shell to /bin/bash in
images/rootfs/etc/passwd. Bash already ships in the image
(images/rootfs/PACKAGES.toml maps /usr/bin/bash with a /bin/bash alias, and
build-source-rootfs-shell-image.ts requires those aliases), so
login -f maker now execs an interactive login bash and completion
returns. Root keeps /bin/sh.

Update the two host tests that assert the maker record against the real
images/rootfs/etc/passwd -- getpwent.test.ts (the getpwent probe) and
demo-login-image.test.ts (the "canonical rootfs data is truthful" guard) --
to expect /bin/bash for maker. The demo-login fixture cases that pass an
explicit /bin/sh to configureDemoLogin are unchanged; they exercise that
helper's shell parameter, not the shipped passwd.

Validation

The changed assertions are string-consistent with the edited passwd by
construction (getpwent deterministically echoes the passwd shell field).
Full browser verification of interactive tab-completion requires booting
the rebuilt shell image; the host tests above are the automated guard and
run in CI.

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