structure: full Columns / Indexes / FKs / SQL Preview UI (DDL UI #6)

Replace the placeholder StatusPage with the working ViewSwitcher
editor. The Structure tab now provides the full DDL editing surface
the audit asked for.

UI layout (adw::ToolbarView):

- Top: AdwHeaderBar with table-name Entry + driver badge.
- Content: AdwViewSwitcher + AdwViewStack with four pages:
    Columns       — one editable row per column: Name (Entry),
                    Type (ComboBoxText with curated per-driver
                    suggestions + free-text), Nullable / PK / Auto
                    (CheckButtons), Default (Entry), Remove (trash
                    icon). Add Column button trailing the rows.
    Indexes       — read-only ColumnView-like rows with Name /
                    Cols / UNIQUE/PRIMARY badge / Remove. Primary
                    index is non-removable (it's owned by the PK
                    column constraint).
    Foreign Keys  — read-only rows with Name / Cols / "→ ref"
                    pointer / Remove. Add Foreign Key button opens
                    an AdwAlertDialog form for name + col multiselect
                    + ref table + ref cols + ON DELETE / UPDATE
                    drop-downs.
    SQL Preview   — sourceview5::View read-only with sql syntax
                    highlighting. Regenerates on every tracker
                    OpsChanged event by calling materialize().
- Bottom: existing ActionBar with Discard / Save / Drop Table.

Editing flow:

- ColumnEdited / TableNameEdited / Add* / Remove* events mutate
  the in-memory model (Rc<RefCell<Vec<…>>>) and then push the
  matching StructureOp + inverse onto the per-tab tracker.
- New mode collapses everything into a single CreateTable op which
  is rebuilt on every mutation; Edit mode pushes per-attribute
  AlterColumn / AddColumn / DropColumn / AddIndex / DropIndex /
  AddForeignKey / DropForeignKey / RenameTable / RenameColumn ops.
  materialize() in StructureChangeTracker handles dialect quirks
  (MySQL alter-grouping, statement ordering).
- StructureTabInput::Refresh tears down the three list views and
  rebuilds them after every mutation. The lists are short enough
  (a handful of columns / indexes / FKs) that full rebuild is
  cheaper than per-row diffing.
- Save runs validate_save() first: rejects empty table name on New
  mode, missing column types, duplicate column names, nullable PK
  columns. Validation errors surface as toasts; structural errors
  from materialize() (e.g. SqliteNotSupported) surface as alerts.
- Discard restores the original DraftColumns from
  ColumnInfo.original (Edit mode) or empties the model (New mode).

SQLite limit handling: rows with original.is_some() get
sensitive=false + tooltip on the Type / Nullable / Default cells
when the driver is sqlite. The trash icon stays enabled (DROP COLUMN
landed in 3.35; older SQLite errors at execute time). Drop Foreign
Key buttons are disabled across the board for SQLite.

What's still deferred to follow-ups:
- MySQL drag-reorder of columns (driver_supports_reorder helper is
  in place; the gtk::DragSource + DropTarget wiring is non-trivial
  and benefits from manual GUI verification).
- update_added_columns_ops surgical replace — for now multiple
  AddColumn ops on the same column may stack; materialize-time
  ordering keeps the final SQL correct, but dirty-count is slightly
  inflated.
- Live drop-down of available tables in the Add Foreign Key dialog
  (today the user types "schema.table" free-text into the entry).

136 tests still green, clippy clean, build green.

Author: datlechin