fix(i18n): add the missing German setup gate strings

[davidpoblador]

Problem

German was missing three keys, so the setup gate rendered in English for German
users:

• Running setup...
• Setup failed.
• Continue to Agent

These are the strings shown while a project's setup script runs and when it
fails, which is exactly the path that just changed in 0.1.78, so it is a bad one
to leave half translated.

Changes

Added the three strings:

"Running setup..." = "Setup läuft...";
"Setup failed." = "Setup fehlgeschlagen.";
"Continue to Agent" = "Weiter zum Agenten";

Also removed "Rebuild" = "Neu bauen";, which existed only in German and is
referenced nowhere in Sources/ or Resources/.

Verification

Audited keys across all five locales. Before: German had 3 missing and 1 orphan.
After, every locale matches English exactly at 248 keys:

ca: missing=[] extra=[]
de: missing=[] extra=[]
es: missing=[] extra=[]
sv: missing=[] extra=[]

plutil -lint passes on all five files.

Not built locally: the ghostty submodule is not checked out in the worktree I was
working from, and building it takes about 15 minutes. This change touches only
.strings content with no source references added or removed, and the release
build will exercise it.

Note, not changed here

All five locales define "GitHub" = "GitHub"; twice, at lines 82 and 206, once
per section. Identical values, so no functional effect. Left alone rather than
touching five files for cosmetics.