fix(theme): dark theme + landing toggle now actually applies

Two bugs:
1. [data-theme="dark"] block was defined BEFORE :root in source CSS,
   so :root cascade overrode it. Moved to AFTER :root and bumped
   specificity to html[data-theme="dark"] to beat plain :root.
2. Theme toggle button only existed in the topbar, which is hidden
   on landing/login/sso-callback scenes. Added a ThemeToggle to
   the Landing header so users can flip theme from the home page.

Confidence: high
Scope-risk: narrow

src/index.css

@@ -9,16 +9,6 @@
    - 1px rules, square edges, monospace operational density
    ===================================================================== */
 
-[data-theme="dark"] {
-  --bp-paper: #0c1322;
-  --bp-paper-2: #1a2740;
-  --bp-paper-3: #243453;
-  --bp-navy: #e6edf7;
-  --bp-navy-2: #d5dde9;
-  --bp-muted: #7a8aa8;
-  --bp-muted-2: #4a5b80;
-}
-
 :root {
   /* Doctrinal hex tokens — declared once, referenced everywhere via var(). */
   --bp-paper: #f5f7fb;
@@ -80,6 +70,16 @@
   --radius-xl: 0;
 }
 
+html[data-theme="dark"] {
+  --bp-paper: #0c1322;
+  --bp-paper-2: #1a2740;
+  --bp-paper-3: #243453;
+  --bp-navy: #e6edf7;
+  --bp-navy-2: #d5dde9;
+  --bp-muted: #7a8aa8;
+  --bp-muted-2: #4a5b80;
+}
+
 /* =====================================================================
    Base
    ===================================================================== */