Alert browser: multi-select hazard types + saveable filter presets

[kellylford]

Summary

Refine the alert-browser digest filtering beyond the current single-select controls. Today the user picks one severity segment (Extreme / Severe / Moderate / All) and one hazard type (or All types). We want richer, persistable filtering.

Desired capabilities

• Multi-select hazard types. Let the user choose several families at once (e.g. Storms + Flooding + Wind) rather than one at a time or all-or-nothing. This directly supports "show me everything from this convective event," which spans several families.
• Saveable filter presets / favorites. Let the user name and save a combination (region + severity + selected hazard types) and quickly re-apply it — e.g. a saved "Severe weather day" preset = All severities + Storms + Flooding + Wind, or a "Winter" preset. Persist to UserDefaults.
• Possibly a quick-apply row of saved presets at the top of the digest.

Notes / open questions

• Interaction with the severity control — do presets capture severity too, or only hazard types?
• Where presets live in the UI (menu, chips, a manage screen) and how they're edited/deleted.
• Accessibility: multi-select and preset controls must be fully VoiceOver-navigable with clear labels/announcements, consistent with the rest of the alert browser.
• Relates to the hazard-classifier families in AlertBrowserService (HazardType).

Deferred — the single-select version is good enough for the initial TestFlight build; this is the next iteration.

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