Add filter recovery controls for empty filter state

[DrumRobot]

Problem

When a persisted message filter (e.g., assistant only) hides all messages in a single-category session, the filter recovery controls (reset/show all) are not visible to the user. This creates a confusing UX where the session appears empty with no way to recover.

Origin

CodeRabbit review on PR #97 (commit 6d447a6) — identified as out of PR scope.

Proposed Solution

• Show a "no messages match current filter" notice with a reset button when all messages are filtered out
• Ensure the category toggle UI remains accessible even when visibleCategories excludes all message types
• Consider adding a session-level indicator showing filtered message count vs total

Acceptance Criteria

• When persisted filter hides all messages, a recovery UI is displayed
• User can reset filter or toggle categories from the empty state
• Storybook story covering empty-filter state