fix: correct dashboard footer date range by sorting on tweet IDs

[ccage-simp]

Description

This PR fixes the date range displayed in the ft viz dashboard footer, which was previously showing incorrect values due to how SQLite sorts raw Twitter date strings.

Before:

After:

The Problem

The dashboard used MIN(posted_at) and MAX(posted_at) to calculate the range. Because Twitter legacy dates start with the day of the week (e.g., Fri Apr 01... vs Wed Sep 30...), SQLite performs an alphabetical sort rather than a chronological one. This resulted in a bogus range (e.g., Fri Apr 01 → Wed Sep 30) that ignored the actual years.

The Fix

1. Accurate Range Query: Updated the query in src/bookmarks-viz.ts to retrieve the earliest and latest bookmarks by sorting on the mathematically-sequential tweet_id (Snowflake ID) instead of the text date strings.
2. Clean Formatting: Added a formatHeaderDate helper to parse the resulting timestamps and display them cleanly as MMM YYYY (e.g., Mar 2018 → Apr 2026) instead of raw 16-character string slices.
3. Visual Polish: Improved the spacing and icons in the dashboard header for better readability.

Testing

• Unit Test: Added tests/bookmarks-viz-range.test.ts to verify that chronological sorting of Snowflake IDs correctly establishes the date range even when the alphabetical order of the timestamp strings is inverted.