refactor(tenants): clean up Visibility tab copy after global-appearance removal

Updates the tenant edit Visibility tab so its labels and helper text
reflect the new tenant-only appearance model. After the global appearance
settings were removed, legends like "Use default color" / "Inherit global
setting" and help texts like "applies the global system appearance" /
"uses its own default theme instead of the global setting" were wrong —
there is no global appearance to inherit from anymore. An empty tenant
field means "use the system default" (hardcoded light theme, no brand
accent, user-theme allowed).

- Info blockquote at the top rewritten.
- "Use default color" → "No brand color" / "Use system default (no brand
  accent)" with updated helper text.
- Default theme placeholder "Inherit global setting" → "Use system default
  (light)" + new helper text.
- User-theme policy "Inherit global setting" → "Use system default
  (allowed)".

i18n: removed the now-orphaned keys ("Use default color", "Using the
default color applies the global system appearance.", "Be careful - this
resets the tenants color", "Inherit global setting", "If set, this tenant
uses its own default theme instead of the global setting."). Added the
new keys in de + en.

No behavior change — only copy. PHPStan, PHPUnit green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

i18n/default_en.json

@@ -425,14 +425,18 @@
   "Appearance": "Appearance",
   "API docs": "API docs",
   "Can view API documentation": "Can view API documentation",
-  "Use default color": "Use default color",
-  "Using the default color applies the global system appearance.": "Using the default color applies the global system appearance.",
+  "Appearance is controlled per tenant. If a field is left at the system default, the app renders in light theme without a brand accent.": "Appearance is controlled per tenant. If a field is left at the system default, the app renders in light theme without a brand accent.",
+  "No brand color": "No brand color",
+  "Use system default (no brand accent)": "Use system default (no brand accent)",
+  "When enabled the tenant renders without a brand accent color.": "When enabled the tenant renders without a brand accent color.",
+  "Use system default (light)": "Use system default (light)",
+  "The theme used for users in this tenant who have no personal preference.": "The theme used for users in this tenant who have no personal preference.",
+  "Use system default (allowed)": "Use system default (allowed)",
   "Inactive tenants are excluded from user access and tenant-scoped data.": "Inactive tenants are excluded from user access and tenant-scoped data.",
   "Status & meta": "Status & meta",
   "Audit": "Audit",
   "Overview": "Overview",
   "Choose color": "Choose color",
-  "Be careful - this resets the tenants color": "Be careful - this resets the tenant color",
   "Code": "Code",
   "Cost center": "Cost center",
   "Department code already exists": "Department code already exists",
@@ -757,10 +761,8 @@
   "Default theme is invalid": "Default theme is invalid",
   "User theme policy is invalid": "User theme policy is invalid",
   "Theme change is disabled for this tenant": "Theme change is disabled for this tenant",
-  "Inherit global setting": "Inherit global setting",
   "Force allow user theme": "Force allow user theme",
   "Force disallow user theme": "Force disallow user theme",
-  "If set, this tenant uses its own default theme instead of the global setting.": "If set, this tenant uses its own default theme instead of the global setting.",
   "Controls whether users in this tenant can choose their own theme.": "Controls whether users in this tenant can choose their own theme.",
   "Tenants can override color and theme behavior in their appearance settings.": "Tenants can override color and theme behavior in their appearance settings.",
   "User theme policy": "User theme policy",