feat(ui): rename nav items — "Sources" becomes "RAG", "Git sources" becomes "Sources"

Owner request (2026-08-28): the two admin-only nav items read like the
same thing, so they are relabeled — the document-catalog link
(#nav-sources, /sources.html) becomes "RAG" and the source-manager link
(#nav-git-sources, /git-sources.html) becomes "Sources".

Phase 48 (48_nav_rename_sources), label-only per the locked decision:
- all six pages (index, sources, git-sources, tuning, document, login):
  the two <a> texts swap; ids, hrefs, hidden defaults, is-active /
  aria-current placement, and nav order (Chat, RAG, Sources, Tuning)
  are byte-unchanged otherwise.
- header.js: comment/docstring label mentions only — the reveal-by-id
  logic is untouched (ship-hidden/reveal contract intact).
- test_git_sources_admin.py / test_mobile_hamburger_nav.py: the two
  suites that asserted the old label text are updated; comment-only
  label fixes in test_shared_header.py / test_nav_consistency.py.
- tests/e2e/test_nav_rename_sources.py: the story E2E (green in
  isolation) — renamed labels + unchanged hrefs/order/markers on all
  six pages, click navigation with the active marker, the anonymous
  ship-hidden contract, and regression guards for the untouched
  controls (#sync-label "Sync sources", viewer #doc-back "Sources").
- All eight surrounding header/nav suites stay green in isolation;
  unit+integration green, app/ coverage 99% (frontend-only change),
  ruff + pyright clean.

Note: per this phase file-level staging, the six page files and
header.js also carry the same-day in-flight owner rework that was
already in the working tree when phase 48 ran (mobile sign-in dropdown
copy, sync button ship-hidden on the Sources page); the label rename
itself is the two-text swap on each page.

tests/e2e/test_git_sources_admin.py

@@ -82,7 +82,7 @@ REPO = Path(__file__).resolve().parents[2]
 APP_URL = f"http://127.0.0.1:{APP_PORT}"
 
 #: The five pages that ship the header (phase 34 contract) — the pages
-#: task 05 gave the admin-only "Git sources" nav link.
+#: task 05 gave the admin-only "Sources" nav link (then "Git sources").
 CHAT_URL = "/"
 SOURCES_URL = "/sources.html"
 VIEWER_URL = "/document.html?source=docs&path=homelab%2Fkubernetes.md"
@@ -307,7 +307,7 @@ def test_admin_nav_link_on_all_five_pages_and_click_navigates(
     expect(page).to_have_url(app_url + CHAT_URL, timeout=30_000)
 
     # The link is revealed (admin) on every one of the five pages,
-    # pointing at the git sources page, labeled "Git sources" — and it
+    # pointing at the git sources page, labeled "Sources" — and it
     # is NOT the current page on the four non-git-sources pages.
     for _name, path in FIVE_PAGES:
         if path != CHAT_URL:
@@ -316,7 +316,7 @@ def test_admin_nav_link_on_all_five_pages_and_click_navigates(
         link = page.locator("#nav-git-sources")
         expect(link).to_be_visible(timeout=15_000)
         expect(link).to_have_attribute("href", GIT_SOURCES_URL)
-        expect(link).to_have_text("Git sources")
+        expect(link).to_have_text("Sources")
         if path != GIT_SOURCES_URL:
             expect(link).not_to_have_class(IS_ACTIVE)