Add customizable home dashboard with per-user pinned apps

- Users can pin/unpin any available service on their home page via a
  Customize mode; preferences persisted via PATCH /api/users/me/preferences
- Time-aware greeting renders the user's display name through React JSX
  (HTML-escaped by design — no dangerouslySetInnerHTML used)
- Added dashboard_app_ids JSON column to users table (migration c7e8f9a0b1d2)
- /settings now routes to a placeholder page

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/user.py

@@ -96,3 +96,24 @@ class UserAdminCreate(UserCreate):
 class Token(BaseModel):
     access_token: str
     token_type: str = "bearer"
+
+
+# ── Dashboard preferences ──────────────────────────────────────────────────────
+
+class DashboardPrefsOut(BaseModel):
+    app_ids: list[str]
+
+
+class DashboardPrefsUpdate(BaseModel):
+    app_ids: list[str] = Field(default_factory=list)
+
+    @field_validator("app_ids")
+    @classmethod
+    def validate_app_ids(cls, v: list[str]) -> list[str]:
+        if len(v) > 50:
+            raise ValueError("Cannot pin more than 50 apps")
+        for item in v:
+            # Service IDs are alphanumeric slugs or UUIDs — no HTML/script allowed.
+            if not re.match(r'^[a-zA-Z0-9_\-]{1,64}$', item):
+                raise ValueError(f"Invalid app ID: {item!r}")
+        return v