test(ui): make ui-visual-update honour its file filter

vitest parses a bare `--update` as taking the next positional as its
value, so `make ui-visual-update UI_ARGS=<path>` handed the path to the
flag and ran with no filter at all: 99 files, every baseline in the repo
re-recorded, any stale one blessed in silence. Two paths were worse
still — the first was eaten and only the second ran.

That is #196's own hazard living in the tool meant to resolve it: the
rule is "refresh the reference your change moved and never one you did
not cause", and the documented way to refresh one refreshed the set.

`--update=true` is the whole fix, with the reason beside it because
`=true` reads like something to tidy away. `make ui-visual` and
`make ui-test` are unaffected — their `$(UI_ARGS)` follows `run`, with
no flag to swallow it — and no other target interpolates a variable
after a boolean flag.

Closes #204

Makefile

@@ -160,8 +160,11 @@ ui-watch: ## Same suite, in watch mode
 ui-visual: ## Run the suite including screenshot comparisons
 	@cd frontend && YJ_VISUAL=1 npx vitest run $(UI_ARGS)
 
-ui-visual-update: ## Re-record the screenshot baselines
-	@cd frontend && YJ_VISUAL=1 npx vitest run --update $(UI_ARGS)
+# `--update=true`, never a bare `--update`: vitest takes the following
+# positional as the flag's value, so `--update <path>` swallows the path
+# and re-records every baseline in the repo instead of the one named.
+ui-visual-update: ## Re-record the screenshot baselines (UI_ARGS=<path> to filter)
+	@cd frontend && YJ_VISUAL=1 npx vitest run --update=true $(UI_ARGS)
 
 ui-setup: ## Install the Vitest browser provider's own Chromium (once)
 	@cd frontend && pnpm install && npx playwright install chromium