audit F19 [observability]: localStorage write failures silently dropped

[intendednull]

File: crates/client/src/storage.rs:264, crates/web/src/palette_recents.rs:58,66, crates/web/src/state.rs:581
Severity: observability
Obvious? yes

let _ = storage.set_item(&format!("willow_{key}"), &encoded); swallows storage write errors (quota exceeded, third-party-cookies-blocked, private mode). User loses unsaved events without warning.

Fix: at least tracing::warn! so devs can repro user-reported "history disappeared".

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Filed by /general-audit @ 6404719 (2026-05-03). master: #567.