feat: add Rose/Amber/Teal color themes, default to Ocean + Square

Adds three new color themes alongside the existing Stalwart/Ocean/
Forest/Violet set so more people can find one they like, and switches
the out-of-the-box defaults to Ocean + square corners instead of the
neutral Stalwart theme + rounded corners (existing users' saved
preferences are untouched, this only changes what a fresh install
starts with).

Verified in the browser: all three new themes render with legible
primary-foreground contrast in both light and dark mode, and the
pre-hydration flash-prevention script in main.tsx (which reads
localStorage before React mounts) now recognizes the new theme names.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/i18n/en.json

@@ -16,9 +16,12 @@
     "switchToDark": "Switch to dark mode",
     "switchToLight": "Switch to light mode",
     "theme": {
+      "amber": "Amber",
       "forest": "Forest",
       "ocean": "Ocean",
+      "rose": "Rose",
       "stalwart": "Stalwart",
+      "teal": "Teal",
       "violet": "Violet"
     }
   },