ui: validity indicator rendering + warning theme colour (ADR-0027 step D)

Adds the `[ERR]` / `[WRN]` validity indicator to the input
row. `App` gains `input_indicator: Option<Severity>` (the
runtime owns its timing — step E) and a pure
`input_validity_verdict()` query that runs `input_verdict`
in simple mode only (advanced mode is raw SQL, ADR-0027 §7).

`render_input_panel` reserves the rightmost six columns of
the input row unconditionally (ADR-0027 §4) — a five-column
label plus a one-column gap — so the typed command never
shifts sideways when the indicator appears or hides. The
label renders only when `input_indicator` is set: `[ERR]` in
`theme.error`, `[WRN]` in the new amber `theme.warning`
(defined for both light and dark themes).

The indicator is not yet wired live — `input_indicator`
stays `None` until the debounce lands (step E). Covered by a
render test and the theme contrast test; the input-panel
snapshot is updated for the six-column reservation.

src/snapshots/rdbms_playground__ui__tests__highlighted_input_all_token_classes_dark.snap

@@ -19,7 +19,7 @@ expression: snapshot
 │                          ││                                                  │
 │                          │╰──────────────────────────────────────────────────╯
 │                          │╭ SIMPLE ──────────────────────────────────────────╮
-│                          ││insert into T values (1, 'hi', null) --all-rows $ │
+│                          ││insert into T values (1, 'hi', null) --all-r      │
 │                          │╰──────────────────────────────────────────────────╯
 │                          │╭ Hint ────────────────────────────────────────────╮
 │                          ││after `insert into T values (1, 'hi', null)`,     │