fix: clear the decorative flag when the upload dialog is handed a different file

The twin of the previous commit, with a sharper consequence. `decorative`
does not merely describe the previous image, it *exempts* it:
`validate-working-copy.ts`'s EVIDENCE_ALT_REQUIRED reads
`Asset.decorative`, so a flag inherited from a discarded divider lets a
meaningful diagram publish with no accessible name at all -- the check
passes rather than catching it. Stale alt text ships a wrong description;
stale `decorative` ships none.

Same terminal-state retry path: tick 장식용 for `divider.png`, have it
rejected, pick `sequence.png`, upload -- and `sequence.png` shipped as
decorative with `altText: undefined`.

Resetting it also re-enables the alt input, so the post-selection focus
call no longer has to ask whether it would land on a disabled control.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/features/tech-log/presentation/studio/components/asset-upload-dialog.tsx

@@ -200,9 +200,22 @@ export function AssetUploadDialog(props: Readonly<{
               // failure: "which file is this text describing" has one honest
               // answer per selection, and `submit()`'s ALT_REQUIRED refusal
               // makes the emptied field impossible to ignore.
+              //
+              // `decorative` is reset for the same reason and a sharper one.
+              // It does not merely describe the previous image, it *exempts*
+              // it: `validate-working-copy.ts`'s EVIDENCE_ALT_REQUIRED reads
+              // `Asset.decorative`, so a flag inherited from a discarded
+              // divider lets a meaningful diagram publish with no accessible
+              // name at all -- the check passes rather than catching it. Alt
+              // text riding along produces a wrong description; this produces
+              // none.
               setState({ kind: "IDLE" });
               setAltText("");
-              if (!decorative) altRef.current?.focus();
+              setDecorative(false);
+              // Unconditional now: the alt field is enabled again by the line
+              // above, so there is no longer a case where focusing it would
+              // land on a disabled control.
+              altRef.current?.focus();
             }}
           />
         </label>