fix: truncate image descriptions (#7699)

* move truncate function to separate file to reuse it

* truncate image descriptions

* show image description limit in UI

* correctly treat null

Co-authored-by: nullobsi <me@nullob.si>

* make truncate Unicode-aware

The strings that truncate returns should now be valid Unicode.

PostgreSQL also counts Unicode Code Points instead of bytes so this
should be correct.

* move truncate to internal, validate in API

Truncating could also be done in src/services/drive/add-file.ts or
src/services/drive/upload-from-url.ts but those would also affect
local images. But local images should result in a hard error if the
image comment is too long.

* avoid overwriting

Co-authored-by: nullobsi <me@nullob.si>

src/misc/hard-limits.ts

@@ -6,3 +6,9 @@
  * Surrogate pairs count as one
  */
 export const DB_MAX_NOTE_TEXT_LENGTH = 8192;
+
+/**
+ * Maximum image description length that can be stored in DB.
+ * Surrogate pairs count as one
+ */
+export const DB_MAX_IMAGE_COMMENT_LENGTH = 512;

src/misc/truncate.ts

@@ -0,0 +1,11 @@
+import { substring } from 'stringz';
+
+export function truncate(input: string, size: number): string;
+export function truncate(input: string | undefined, size: number): string | undefined;
+export function truncate(input: string | undefined, size: number): string | undefined {
+	if (!input) {
+		return input;
+	} else {
+		return substring(input, 0, size);
+	}
+}