fix(updater): stop false-positive "update available" loop when commit is unknown

Dockerfile never received DASHCADDY_COMMIT at build, so /app/VERSION held
'unknown'. _isNewer then treated same-version-different-commit as newer,
making the auto-updater rebuild the container indefinitely (each rebuild
still produced commit='unknown').

- self-updater._isNewer: normalize commits; treat unknown/null/empty as no
  commit info and fall back to pure version comparison
- self-updater._autoCheckAndApply + routes/updates: refuse to apply when
  local version >= remote version (belt-and-suspenders)
- update-management.js: hide '(unknown)' from version label
- Dockerfile: COPY VERSION instead of writing from build arg
- VERSION: committed placeholder ('dev'); scripts/release.sh now writes
  the real short SHA into the tarball's VERSION before tar-ing, so every
  published release ships with an accurate commit

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dashcaddy-api/Dockerfile

@@ -13,8 +13,10 @@ COPY src/ ./src/
 COPY routes/ ./routes/
 COPY openapi.yaml ./
 
-ARG DASHCADDY_COMMIT=unknown
-RUN echo "${DASHCADDY_COMMIT}" > VERSION
+# VERSION file holds the short git SHA the image was built from. Committed as
+# 'dev' for source builds; the release script (scripts/release.sh) overwrites it
+# with the actual commit hash before tarballing each release.
+COPY VERSION ./
 
 # Note: Running as root because container needs Docker socket access
 # (which is root-equivalent anyway). Socket access required for container management.