feat(web): customizable placeholder, footer text, and color theme via BOR_* env vars

BOR_INPUT_PLACEHOLDER / BOR_FOOTER_TEXT / BOR_THEME (+ the indigo.css example theme); authoring guide: frontend/assets/themes/README.md, docs: README 'Customizing the look'.

app/api/config.py

@@ -1,5 +1,7 @@
 """Public app metadata (display name + version) for the frontend brand
-layer, plus the phase-59 docs-push flag (the "Save as doc" gating)."""
+layer, the phase-59 docs-push flag (the "Save as doc" gating), and the
+phase-62 UI customization strings (composer placeholder, footer line,
+theme file name)."""
 from __future__ import annotations
 
 from fastapi import APIRouter, Depends
@@ -12,12 +14,18 @@ router = APIRouter(tags=["config"])
 @router.get("/config")
 def app_config(settings: Settings = Depends(get_settings)) -> dict[str, str | bool]:  # noqa: B008
     """Public app metadata for the frontend brand layer (phase 39) +
-    the phase-59 ``docs_repo_configured`` flag — the chat page's
-    "Save as doc" button gating, surfaced the way ``app_name`` is
-    (the SAME boot fetch, no new network surface). Inert false while
-    ``BOR_DOCS_REPO`` is empty (the feature is off, D3)."""
+    the phase-59 ``docs_repo_configured`` flag + the phase-62 UI
+    customization keys (``input_placeholder``, ``footer_text``,
+    ``theme``) — all display strings, the SAME boot fetch (no new
+    network surface) and the same public posture as ``app_name``
+    (no secrets). Values are passed through verbatim: the frontend
+    brand layer treats an empty string as "keep the template default"
+    (the unset => byte-identical contract)."""
     return {
         "app_name": settings.app_name,
         "version": settings.app_version,
         "docs_repo_configured": settings.docs_configured,
+        "input_placeholder": settings.input_placeholder,
+        "footer_text": settings.footer_text,
+        "theme": settings.theme,
     }