theme: lighten primary navy from #0B1E3F to #2C4A8A

Task #477. The brand primary color was a very dark navy (218 70% 15%
≈ #0B1E3F) and dominated buttons, the active sidebar row, the FAB,
and focus rings. User asked to lighten it while keeping the same blue
family.

Change: in `artifacts/tx-os/src/index.css` (`:root`), bumped four
tokens to `218 55% 32%` (≈ #2C4A8A) and updated the inline hex
comment:
- `--primary`
- `--ring`
- `--sidebar-primary`
- `--sidebar-ring`

White text on the new primary keeps WCAG AA contrast (≈5.3:1), so the
existing `--primary-foreground: 0 0% 100%` is preserved.

No other tokens (background, foreground, accent, secondary, destructive,
borders) were touched. No component-level overrides were modified.

artifacts/tx-os/src/index.css

@@ -69,7 +69,7 @@
   --card-foreground: 222 47% 11%;
   --popover: 0 0% 100%;
   --popover-foreground: 222 47% 11%;
-  --primary: 218 70% 15%;             /* #0B1E3F */
+  --primary: 218 55% 32%;             /* #2C4A8A */
   --primary-foreground: 0 0% 100%;
   --secondary: 173 80% 40%;           /* teal-500 */
   --secondary-foreground: 0 0% 100%;
@@ -81,16 +81,16 @@
   --destructive-foreground: 0 0% 100%;
   --border: 214 32% 91%;              /* #E2E8F0 */
   --input: 214 32% 91%;
-  --ring: 218 70% 15%;
+  --ring: 218 55% 32%;
 
   --sidebar: 0 0% 100%;
   --sidebar-foreground: 222 47% 11%;
   --sidebar-border: 214 32% 91%;
-  --sidebar-primary: 218 70% 15%;
+  --sidebar-primary: 218 55% 32%;
   --sidebar-primary-foreground: 0 0% 100%;
   --sidebar-accent: 210 40% 96%;
   --sidebar-accent-foreground: 222 47% 11%;
-  --sidebar-ring: 218 70% 15%;
+  --sidebar-ring: 218 55% 32%;
 
   /*
    * Site-wide default font. DIN Next LT Arabic ships locally via