Keep the dashboard temperature tooltip inside the chart

[kaysond]

Closes #950

What was happening

The dashboard temperature chart runs in sparkline mode, which forces legend.show = false, so the
only way to tell which line is which drive is the shared tooltip. That tooltip grows one row per
device, and two things then conspire against it:

1. ApexCharts only keeps the top half of the tooltip inside the chart. Position.moveTooltip()
   clamps with if (tooltipRect.ttHeight / 2 + y > gridHeight), so the bottom of the tooltip can
   sit up to ttHeight / 2 below the chart.
2. The card wrapping the chart is rounded overflow-hidden, so whatever hangs below gets clipped.

Measured on a 1280×900 viewport with 7 drives, hovering the series the way the reporter's screenshot
shows: the tooltip is 266px tall and 119px of it is cut off.

The fix

• pin the tooltip with tooltip.fixed = { enabled: true, position: 'topLeft' }. That skips
  moveTooltip() entirely (if (!ttCtx.fixedTooltip) this.moveTooltip(...)) and anchors the tooltip
  to the top-left of the chart, so it is laid out downwards from a known point instead of around the
  cursor.
• drop overflow-hidden from the chart card. For a large number of drives the tooltip is simply
  taller than the chart, and clipping it is what made it unreadable. The temperature card is the last
  block on the dashboard, so a tall tooltip just extends into empty page space.

Both are needed: fixed alone still clips from 8 drives up, because the tooltip becomes taller than
the 275px chart.

Measurements

Driven with headless Chromium against a production ng serve, hovering the chart and measuring the
tooltip against the clipping ancestor. "clipped" is how many pixels of the tooltip were cut off:

drives	tooltip height	before	fixed only	this PR
7	266px	119px clipped	0	0
8	299px	clipped	12px clipped	0
12	431px	clipped	144px clipped	0
16	563px	clipped	276px clipped	0

Also checked at a 414×896 viewport (7 drives): tooltip fits, nothing clipped.

Not fixed here

The palette in _prepareChartData() has only 6 colours, and ApexCharts recycles them, so from 7
drives up two lines share a colour. That is a separate (and more debatable) change — the tooltip
markers have the same problem, and fixing it properly probably means generating colours rather than
listing them.

AI disclosure

Per AI_POLICY.md: this change was written by Claude Code (Opus 5). It reproduced
the bug, produced the numbers above, and made the change. Everything ran in a node:24-trixie
container driving headless Chromium via puppeteer against ng serve --configuration production, with
the API responses stubbed at the network layer so the drive count could be varied; nothing was
installed on the host.

Automated verification performed:

• reproduced the clipping at 7 drives and confirmed it is gone after the change, at 7/8/12/16 drives
  and at a phone-sized viewport
• npm run build:prod and npx ng test --watch=false --browsers=ChromeHeadless --code-coverage
  (87/87)

No unit test was added: the change is chart configuration, and a test asserting the option is set
would only restate the diff. The behaviour that matters is geometric and was checked in a browser.

That is the extent of the verification behind this PR as opened.