Add stable GUI test IDs

[Hrabovszki1023]

Closes #35

What changed

• adds a normative data-testid registry to the accepted Web UI v0.1 contract
• defines reversible, collision-free UTF-8 percent encoding for dynamic barcode components
• adds stable rack, health, MQTT, topology, module, heatmap, cell, identity, freshness and error selectors
• keeps selector identity based on module barcode and zero-based cell index rather than rack position
• preserves selectors across full-page and HTMX fragment rendering, responsive layouts, and unavailable/stale/invalid states
• updates browser tests to consume the new machine-facing interface without adding OKW or Selenium dependencies to product code

Verification

• python -m pytest -q — 203 passed, including Playwright desktop and narrow viewports
• python -m ruff check . — passed
• python -m mypy src tests — passed
• git diff --check — passed

Scope

No REST, MQTT, polling, parsing, measurement, calculation, safety-color or visible-layout behavior is changed.

Before merge

Keep this PR in draft until the rendered Docker version has been sampled on the Proxmox host and the expected data-testid attributes are confirmed in the live HTML.