User walkthrough updated + Plot timestamp bug fixed

[tylerd0307]

Overview

Devices would send timestamps with missing leading zeros (e.g. T2:5:56 instead of T02:05:56). MongoDB's $dateFromString rejects these silently, causing those packets to be dropped from all date-range queries and the earliest/latest date display.
The user walkthrough had a bug where the Multi-Axis toggle disappeared during its own step, and the content needed updates to reflect recent UI changes.

Changes

Server (server.js):

• Added a fixTimestamp helper that zero-pads non-padded ISO time components so any device firmware format is handled consistently.
• Replaced the $dateFromString MongoDB aggregation in /data with a Node.js find + filter approach, using fixTimestamp for precise filtering.
• Replaced the /date-range aggregation with _id-sorted lookups and Node.js normalization, so the earliest/latest packet dates are correct even for devices with non-padded timestamps
• /date-range now returns timezone-naive device local time strings instead of UTC ISO strings, preventing a double timezone shift that was causing query windows to miss data for some collections

Client (index.js):

• Removed the UTC-to-local conversion in setDateBoundsForSelection since the server now returns device local time directly
• Removed the trailing Z from the client-side fixTimestamp so that time_local values display on the plot exactly as the device recorded them, with no browser timezone shift applied (per Chet's request to display local time as-is)
• Fixed the Multi-Axis toggle disappearing during its walkthrough step
• Updated the "Confirm Source" walkthrough step text to reflect that the toolbar button now shows the selected device name
• Added walkthrough steps for Download CSV and Import/Export Workspace

Notes
Devices with properly zero-padded timestamps are unaffected