chore: root .editorconfig and hardened .gitignore (closes #15)

Three root-level dotfiles diverged from the org models. Two are fixed
here; the third is deferred for a reason spelled out below.

Move backend/.editorconfig to the repo root. The file is byte-identical
to the org model, so this is a pure relocation with no content change.
It carries root = true, which one level down was actively harmful: it
stopped editors walking further up, leaving the entire frontend
(src/, index.html, vite.config.js, nginx.conf, script/) with no
indentation settings at all. At the root the same file covers the whole
tree, backend included, so the subdirectory copy is redundant.

Replace .gitignore with the org model verbatim, then re-append the two
repo-specific entries the model does not carry: dist/ (Vite output) and
*.log. This adds the OS entry Thumbs.db, the entire Editors section
(*.swp, *.swo, *~, *.bak, .idea/, .vscode/, *.sublime-*), and the
Environment / secrets section (.env, .env.*, *.pem, *.key).

The secrets section is the substantive part. The backend loads .env via
godotenv and only backend/.gitignore ignored it, so a .env at the repo
root was untracked but unignored -- one git add -A away from being
committed. Policy allows no exceptions there.

Not done here: excluding .git from .dockerignore. Both images read git
metadata at build time. Dockerfile.backend has an explicit
COPY .git /repo/.git feeding git describe in backend/Makefile, and the
frontend Dockerfile's make check runs vite, whose config calls
git rev-parse at config-eval time. Ignoring .git breaks both builds
outright rather than degrading them, and decoupling them from git
metadata belongs to the Dockerfile rework in #17. Deferred deliberately,
not overlooked; tracked separately so it is not lost.

No tracked file becomes ignored by the new patterns: git ls-files
differs only by the .editorconfig relocation, and check-ignore over the
full tracked set matches nothing.

.editorconfig

@@ -0,0 +1,12 @@
+root = true
+
+[*]
+indent_style = space
+indent_size = 4
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[Makefile]
+indent_style = tab