fix(installer): database page lost its Username and Password boxes

Reported from the Server 2019 test: the Existing database page rendered a
truncated "Username:" label and no input boxes at all below it, so there was no
way to enter connection details.

CreateInputQueryPage lays its fields out BELOW the description text. The
description had grown to include a five-line CREATE DATABASE / CREATE USER /
GRANT block, added so a DBA could be handed the exact SQL. With five fields
underneath, the last two fell past the bottom of the page surface, where they
are simply not drawn - no error, no scrollbar, just missing controls.

The description is back to three lines. The SQL moves to
docs/INSTALL-WINDOWS.md, which is where someone would look for it anyway and
where it can be copied without being retyped from a wizard page.

Wizard page descriptions are a fixed budget: anything long enough to be worth
reading twice belongs in the guide, not on the page.

docs/INSTALL-WINDOWS.md

@@ -45,6 +45,21 @@ Two options. Pick before you start, because they ask different questions.
 If you are unsure: if nobody has given you database credentials, you want the
 bundled option.
 
+### SQL for your DBA (existing-MySQL option only)
+
+The installer does not create the database or the user - it never needs
+administrative rights on your database server. Ask your DBA to run:
+
+```sql
+CREATE DATABASE shopdb_flask CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+CREATE USER 'shopdb'@'%' IDENTIFIED BY '<a password you choose>';
+GRANT ALL PRIVILEGES ON shopdb_flask.* TO 'shopdb'@'%';
+FLUSH PRIVILEGES;
+```
+
+The `utf8mb4` charset matters - the default on older servers is `latin1`, and a
+latin1 database mangles any non-ASCII text you store.
+
 ---
 
 ## Installing