Docs: capture the Svelte effect poisoning failure mode in ipc/CLAUDE.md

The original Gotcha covered the cosmetic symptom (literal 'null' in the UI) but not the silent-failure variant that cost most of the F8 debugging session: when a bad `=== undefined` check sits inside a Svelte 5 `$effect`/`$derived` and the downstream code throws on `null`, the throw corrupts the reactive graph for sibling effects on the same component. State writes happen but dependent effects stop running, and any `{#if state}` block stays stuck. Documents the symptom signature and the typical suspect pattern (typed numeric/Intl helpers receiving an optional field) so the next agent can recognize it in <30s instead of <a day.

Author: vdavid

apps/desktop/src/lib/ipc/CLAUDE.md

@@ -136,6 +136,18 @@ render path) accepts `null` as a real value and surfaces literal `"null"` in the
 `dc5f0b47`) flipped the shape across the board; size and date columns rendered the literal text `"null"` until consumers
 were switched to `!= null` checks.
 
+**The non-obvious failure mode (lost a day to it):** when the bad `=== undefined` check is inside a Svelte 5 `$effect`
+or `$derived` and the downstream code throws on `null` (`formatDateTimeParts(null)`, `null.toLocaleString()`,
+`Number(null) + something` ending up `NaN`-poisoning a guard, etc.), the throw doesn't surface as an error in the UI
+or the console — it silently corrupts the reactive graph for *sibling* effects on the same component. The classic
+signature: a `$state` write happens (you can see the new value if you read the variable synchronously after assigning)
+but a separate `$effect` that should re-run on that change *doesn't* re-run, and any `{#if state}` block that depends
+on it stays stuck on its previous truth value. We hit this on F8 (delete dialog) after a volume switch: the listing
+effect threw on a `null` `modifiedAt` from an SMB/MTP entry, poisoned the graph, and then `showDeleteDialog = true`
+silently failed to mount the dialog. The fix is always the same — find the `=== undefined` site that throws on `null`
+and switch it to `== null`. Suspect every site that calls a typed function (`Intl.*`, `(n: number) => …`) with an
+optional field as input.
+
 ## Architecture
 
 The Rust side splits into: