fix: improve foreign stock chart readability

[AlexTzib]

This is a small follow-up to make the foreign stock history graph easier to read without changing the rest of the foreign stock flow.

What this changes:

• plots the chart using real timestamp spacing instead of treating every sample as the same distance apart
• adds subtle vertical markers when stock appears after being empty, so restock moments are easier to spot
• reduces the bottom axis labels and keeps them horizontal, which should make the chart less crowded on smaller screens
• adds a short chart context label so it is clear whether the chart is showing the full recent history or a shorter high-activity window

Why:
The old graph could be a bit misleading because a 3 minute gap and a 30 minute gap were drawn the same way. That made the history harder to reason about, especially around restocks. This keeps the existing chart and data, but presents the time axis closer to what actually happened.

Local verification:

• Checked the changed chart APIs against the pinned fl_chart 1.0.0 package in the local pub cache.
• Ran git diff --check.

I could not run Flutter analyzer/build locally because flutter/dart are not available on this PATH, so I expect GitHub CI to be the real compile check here.